No, it's not 2-tiered, those stats you mention - int, str and dis - are randomly generated on all raid gear. There are several variations with more or less of these stats. In terms of "tier" they are all in the same +30 light tier and they have all of the same bonuses (i.e. increase grenade or melee regen, etc) just different stats. Obviously, if the random stat roll gives you better stats, you'll take the better ones. I just finished leveling my 2nd set of raid gear and that same day got another set of hezen lord boots which had the same stats (Int and Dis) but the new boots had 6 more int and 8 more dis...needless to say, I wasted leveling my 2nd boot set and started leveling up the new boots since they have the better stats.