the buffer is a fine-tuning point for the gas system. the rifle and standard carbine buffers should work with nearly any AR-15, if you are noticing harsh recoil, bolt bounce (where the bolt carrier group slams forward with enough for that it "bounces" off the face of the breach and slightly unlocks, only to lock itself again under spring pressure), or particularly loud/unpleasant blast effects (typical of short barrels), you should then use the buffer to fine tune the action.
that being said, an H buffer is already very common on carbine length gas systems with fairly typical gas ports and a normal carbine spring and no suppressor. an H2 buffer is typical for an especially short barrel, usually under 13" assuming the gas port is not modified, but i see H2's being used on 16" barrels with carbine length gas systems to help balance things out. it's very easy to swap between the buffers, but be sure that you test and determine if it's really solving a problem. the action is meant to be fairly violent when in operation, but not excessively so.