Crafting: Utility slots versus Offense/Defense slots?

Better for what? That's not a rhetorical question -- optimization needs a goal and there are many goals to go for in a warrior build. Increasing guard pen might be perfect for one build and a waste on another.

In a very broad sense, here is an overall crafting priority. It just speaks to the benefit-per-unit-material ratio of each buff, not to what you should pick in all cases:

3% Flanking Dmg > 1.75% Attack > 3% Armor Pen > 1.75% Crit Chance = 1.75 Dexterity (rogue) >1.75 Willpower > 1.75 Magic (mage) = 1.75 Strength (warrior) > > 1.75 Cunning > 3% Crit Dmg > 3% Barrier Dmg > 3% Guard Dmg

Note that that priority was created before the three content DLCs came out, so some items should move up and some move down, as new materials, schematics and gear were added.

Armor Pen shouldn't be so high. Or better to say, Armor Pen is high priority for as long as your average damage is about the same as average enemy armor, which is just in the early game. Once you get to Skyhold, there are better options for the slots and Armor Pen should fall way down the list below Crit Dmg.

If you are optimizing for damage, you want the buffs that have the fewest restrictions. Guard penetration doesn't do a thing for you if the enemy doesn't have guard. Likewise for Barrier Pen (unless you have Walk Softly enabled). That's why they are towards the end of the priority list. Attack and Crit/Crit are going to do so much more for your damage improvement than any kind of anti-protection bonus. All classes should invest in Attack and Crit/Crit to some extent.

For a warrior, after Attack and Crit/Crit, go for max health. Max guard is calculated from max health, so you get 2-for-1 benefit on investing in max health.

Then the rest is dependent on your play style. Are you more of an offensive style or defensive style player? DPS or tank? Choose buffs accordingly.

If you need to choose between Utility and Offense crafting, first consider your resources. If you have a ton of mats that boost Strength, but few that boost Attack, go with Utility. If you've mats enough for everything, consider which schematic yields the highest combined offensive stats. If one schematic has 10 Utility slots and another has 6 Offense slots, all else being equal, use the 6 Offense, because +6% Attack is better than +5% Attack (Utility to Strength), even if the Utility schematic gives you +10% guard damage bonus, because the usefulness of that combat stat is low.

Compare to a rogue optimizing for crit/crit damage. For a rogue, Utility for Dexterity is almost always the best choices, because you get +1% Crit Dam for ever +0.5% Attack. It's one of the best exchange ratios in crafting.

Here's an even simpler rule of thumb for each class:

Mage

Offense: Attack, some Critical Chance
Utility: Magic
Defense: Max Health

Rogue

Utility: Dexterity
Offense: Attack, Critical Chance then Critical Damage
Defense: Max Health

Warrior

Offense: Attack, some Critical Chance
Defense: Max Health
Utility: Strength

NOTE: Gear, like the Ring of Doubt, or abilities, like Knife In The Shadows, can remove the need to buff Critical Chance.