Which game is harder, ME or DAO?

That's a really hard question to answer. It is going to depend more on what types of games you have played in the past. I can give you my experience from someone who played ME1-ME3 before ever playing DAO.

I didn't think Mass Effect was too bad when I started out, but that was progressing from Veteran to Hardcore to Insanity with a single character going NG+ when I unlocked Hardcore and Insanity. Didn't really know what I was doing, but had played a lot of shooters in the past. Later on when I started playing Insanity from level 1 it felt a good deal harder since you are fairly weak in the beginning, but by level 20 you start to feel alright, and by late game you will be a lot more powerful (combination of talents and especially gear). Since Insanity isn't even unlocked on a new profile, this isn't really an issue.

ME2 is probably the hardest of the trilogy on Insanity, at least if I try to say something like an average over levels and classes. Hardcore and Insanity add something to every enemy that makes it a bit more challenging than lower difficulties.

ME3 isn't too bad on Insanity, especially post patch buffs and especially with DLC equipment.

As for DAO, it was actually the first RPG of that type I had ever played and it felt like I kept getting my ass kicked the first time on Normal. That was probably in no small part because I didn't really understand the mechanics. My first Nightmare run felt easier than that very first run after learning how the game worked, and I have since solo'd that game with the major styles.

Basically my point is just that there isn't really a lot that directly translates between these two series, so it is hard for me to predict if you will find it harder than DAO or not.

I have played both. I would say for the first playthrough, ME1 was easier. DAO is far more team oriented and it took me a long time to configure my team tactics and the talents for my team to compliment each other nicely. With ME1 I felt it was all up to me. While there is a 'pause and command' mechanic in ME1, its not like you can queue up mutliple actions. There are no sustained talents in ME1, just passive and active ones. There are powers and biotics, but the game doesnt let you queue up multiple actions and preserves the dynamic aspect of it being a shooter.

That being said, you might need to micro manage your squad in some instances as they will shoot at an emeny regardless of any object or wall standing in the line of sight. So they have xray vision but not the sense to not shoot at the wall.

I think the mechanics take some getting used to and I found the learning curve of ME1 to be much easier than DAO. I know you dont want spoilers but would warn of the following:
1- ME1 encourages a paragon/renegade morality system that rewards being strong in one rather than being medium in both
2- Talk to your squad and get to know them to unlock additional missions or tasks

In DAO you could make any decision you wanted at any point in the story. ME1, and all ME games for that matter, use your past choices to lock you into a specific path by the end of the game, and following the advice in (1) and (2) will be the difference between dealing with a tough situation successfully vs. having undesirable (or desirable?) outcomes forced on you. I mention this because you can pick up the game mechanics quickly, but going back to play half the game over again: not so much.

There is a bit of a misconception about the paragon/renegade system in ME1.  Dialogue options are unlocked more often based on where you have assigned your XP points than where you've made choices in the past.  If you're wanting to make more renegade dialogue choices, you need to place XP points into the Intimidate category, if you want to make more paragon dialogue choices, you need to place XP points into the Charm category.  Sure, a number of slots open up in each of those categories up as you make choices using that allignment, but there are ample choices to be made to unlock all 12 slots in both categories in a single playthrough.  Many people, however, do not like investing the XP points into both charm and intimidate because they tend to want to make their Shepard stronger in the areas of combat, regen/shields/barriers, and powers.  By not assigning enough points into either charm or intimidate is eventually what limits Shepard to a singular alignment by the end of ME1.  I have, however, played the game through many, many times with various balanced/neutral Shepards and received the maximum import bonus for both when I take the file into ME2... all without any glitching. 

ME2 certainly is more "forcing" about keeping to a singular alignment.  However, I have also successfully played an ME2 Shepard who was pretty equally both paragon and renegade and managed to obtain and retain every squad mate's loyalty.  That playthrough, however, took some serious planning regarding the timing of missions and the triggering of the infamous arguments.

ME3 actually has very few places where alignment matters... Passing the checks is more about having sufficient general reputations points (i.e. having done a certain percentage of the missions).

There are achievements available for playing most of the game with individual squad mates (one achievement for each).  With the soldier, you'll need to be sure that you have both electronics and decryption on your squad at all times... that precludes using, say, an Ashley and Wrex team (since neither has either one) or, say, a Liara and Wrex team (since Liara only has electronics and no decryption).  You will also want to get those two skills up to the hard level as quickly as possible since some of the crates you need/want to open to loot require hard level decryption or electronics.  I find Kaidan a very useful squadmate... but looking at your list, you do seemed focused on not using the humans and you don't seem interested in Garrus (or have not recruited him).  Of your list, my preference leans ever so slightly to Tali and Liara... but it is a very miniscule preference (Tali and Wrex are also very good together... and Wrex's quips can be a lot of fun).

For getting the achievements initially, I rolled the entire game through the first time with Ashley and Kaidan.  On my second playthrough, I rolled with Garrus and Tali (NG+ soldier on insanity) and then rolled with Wrex and Liara on my third playthrough (using an infiltrator for that one since the infiltrator has his/her own decryption and electronics).

An advantage with changing them up though is that you can tailor their powers according to the missions you're doing (of course that involves looking at the wiki though, which does give a recommendation based on the sorts of enemies you'll be facing).  Once you get them all leveled up and get yourself familiar with how each on works though, any of their powers can be used pretty much to good effect on any mission and with any Shepard.

Everyone else will know more about the combat aspect of that so I'll leave it to them.
From a character perspective ME1,2 and 3 work differently when it comes to dialogue.  In ME1 your squad is programmed to give you their perspective about a decision you're gonna make, but they will always have a different opinion to each other.  Like an angel and a devil on your shoulder.  That get's confusing when one run Garrus tells you he's fine with something you're about to do and then the next he's against it.
So essentially it doesn't matter who you take from a narrative point of view as they rarely have anything useful to say.  The only exception is to take Liara to the big quest on Noveria that starts when you leave the Noveria port in the mako.  But hey you can do it without her, if you're character isn't her mate then leave her behind.

In ME2 and 3 they fixed this and gave them permanent morals, but 2 has very little banter anywhere at all sadly.  Finally in 3 they added loads of banter and it's really worth mixing it up and hearing them chat, even to the point where if you've slept with both characters they bicker with each other.  Great stuff.

There is some "free banter" in ME1, but you'll have to remember to turn and click on the squad mates to trigger it, unless you're in an elevator. Most of it is on the Citadel... as you're wandering about, you may here one or the other squad mate will utter "huh" or something similar... that's a cue to stop and click on that squad mate to enable them to make a quip. One main mission planets, there are a few quips you can trigger that way that don't have the "huh" cue... so on those planets, whenever you're out of the mako, do occasionally just click on the squad mates to see if they have anything to say. At decision points during some of the missions, some squad mates will chime in with information on that decision. In this case, they will present opposing views from each other... and their views may sometimes change depending on which squad mates you have with you. It's also worthwhile to be sure to tour the elevators on the Citadel will different combos of squad mates. What news items and quips trigger is somewhat random, so it takes a bit of effort to hear them all.

In ME2 and ME3, the quips become automatic, so you don't have to think about turning around and clicking on your squad mates to trigger them. The quips do change depending on which two squad mates you bring (I'm still unlocking ones I haven't heard before).

I dont know in what platform you play, but in regards to dragon age 1, the pc version is a lot harder than the consoles. I never played DA1 on a console, but Ive heard that aside from the AI being easier on the player, the main character and companions cause way more damage (and suffer less damage).

Mass Effect is very easy to learn, but a little hard to master. There is casual difficulty that makes things very easy. In mass Effect 3 there's even a narrative mode that is basically win mode (its actually quite hard to die in it).

ME1 is clunky, but not really hard.

ME2 is a little bit hard, specially if you've become accustomed to the style of ME1 (the game changed quite a bit in its mechanics).

ME3 is easy to learn, and if you've played ME2 then you already know about 90% of it. Combat is a lot of fun, easy to control and the character can evolve in many different directions. You can taylor shepard in the exact way you want.

I do suggest that you access the ME wikia and study the six different classes.

I found ME1 way, way, way harder than DA:O. I have never played anything with aiming before, and I was on PC. The first tough battle in ME1 I could not even win on Casual and had to call my husband to fight past it. No problems on the second run. in DA:O I had no problems staring on Normal. I found the 2+ titles of both series too easy on Normal, but "Am too lazy for that" for next level of difficulty, though I've tried it, and, yeah, I can play it, but dying is boring.

EDIT: I also always rotated chars in MET, but by some reason went with a set party unless I have to shuffle in DA franchise. Probably because the roles are more defined.