I don't know the exact formula, but you'll always want to wear the armor rated for your class. I.E. don't wear marauder gear as a jugg, even if the stats are better since it isn't heavy armor. Armor rating on moddable gear is determined by the armoring in them, so higher level armoring should have better stats anyway. For the most part, I haven't noticed non-moddable gear that had a significantly higher armor rating with reduced stats (maybe reduced tank stats for higher DPS stats) unless it was a different armor type (heavy vs. medium). Either way, you won"t have the problem at end game and as long as your gear is within 5 or so levels of your character, you really won't have any problems running any content while leveling.
O.K., thank you.