Lower AP is better. AP is essentially what armor save you ignore. For example, Sisters of battle and Space Marines both have a 3+ armor save from their power armor. So for every wound they would take, you can roll a d6, an on a 3 or higher that wound is negated. However, if its from a weapon that's AP 3 or better (better meaning lower, so 2 or 1), then the armor save cannot be taken.
Also, AP 2 and AP 1 get bonuses against damaging vehicles, with AP 1 getting a larger bonus than AP 2 (because there's no such thing as a 1+ armor save)
You've got the right idea on strength.