Low offer the most realism 20 - 40 bbs per mag. Best suited to milsim. You'll be heavily out gunned in skirmishes
Mid caps offer the best middle ground with realism but enough rounds to stay in a fight. 80 - 200 bbs per mag (dependson brand). Can be used in both local skirmish and milsims.
High cap have the most bbs per mag 400-650 bbs. Can't use at milsims good for players who constantly pound the trigger or saves on money as you need less to play. You'll also have to wind a wheel constantly to get through the mag i.e every 75-120 rounds
I personally only buy mid caps as I hate the rattle and having to wind when running or shooting with high caps