Better to get a level 1-2 Rare / ultra rare weapon or a level 7-10 common / uncommon weapon?

The question is what happens if you buy 5 of the most expensive and the best you get is a crap rare gun like what Revenant used to be in ME3? UR drops in ME3 were at 7.5%, so I wouldn't bet my money on trying to get those early.

In general, it was always suggested level up the uncommons and rares first. Besides, you will also get a lot of consumables that you will really need, while the expensive packs do give you better consumables, you won't have enough as the missions are not giving you enough credit to pay for themselves.

You should not be buying expensive packs until you can do gold, a general rule that should still apply from ME3! Unless you really enjoy gambling, but overall it's very poor value mostly because you'll be behind on consumables, and using max level amps on bronze/silver is a big waste.

I have charger X (common) and hurricane I (UR) and they are pretty much the same dps vise.