My 2 weapon ranger is lvl 4 and has a 19 Str and 17 Dex. He has +1 hide armor and Two Weapon Defense Feat. His AC is 20, while the main defender in our group has AC 23. I would expect his AC difference from the defender to remain about the same as well level from this point onward, In fact it may narrow as my Dex will go up but the defender will never get any stat bonuses to his AC as we progress. This seems acceptable to me given that I will be dishing out more damage, I need to make use of his marking ability or stay out of harm's way as much as possible while still dishing out the love.
The defender will catch up / surpass when you start getting the +4 armors that jump. Assuming a light shield, and assuming you both take the armor specilization feat at 11, assuming the light armor wearer starts with 18 INT or DEX and bumps it every time, comparing Hide to Scale (you can adjust to leather or Plate easily by + or - 1, with an extra -1 for leather above level 11 for no feat).... Also assume you get +1 armor at level 1, +2 armor at level 6, etc. That may not be realistic but it's a nice simplification.
From level 1-7 the Hide Armor wearer will be 1 behind (3+4 = 7 to start vs. 7 + 1 (shield) = 8 to start). At level 8 you catch up from that +1 stat. At level 14 you pull ahead by one from your second +1. At level 16 you both get your first set of +3 armor, but this gives him an extra +2 because heavy armor jumps for exactly this reason. The result is that heavy is now up by 1. At level 21 you catch up again due to ability score boosts, and at 26 when you get +6 armor heavy jumps up by +2, a lead you'll cut down to +1 at level 28.
Leather vs. Scale is -3 by the end, Leather vs. Plate is -4, etc. You can pick up Two Weapon Defense possibly too, to offset the difference (and they can take heavy shield, or you could be a staff wizard, or you could have started with 20, or gone Demigod and boosted it even more, etc.).
Now if ability is only say +2 to start and only goes up at 11 and 21... then the Hide armor guy is 6 behind at the end (Leather is -8). Is that a big deal? I don't know, I haven't played at level 30 yet.
