I think the general concensus depends on what exactly you want out of your system. it sounds like you're trying to get something really impressive, a "can handle anything" system, so short answer is, I think you'd be better off with a second battery.
every amp has capacitors (sp? it's late...) already in it from what I understand, just not very big. they just store energy from your alternator, and your amp reaches into the cap when it needs power. if you're playing music that has a LOT of bass (like most hip hop/rap), and the cap is constantly draining and recharging, then it's really not doing much but being a middle man for the amp.
a second battery will, in the end, give you more power/backup power than any caps really would (to a certain extent...but I'm trying to be realistic here).
other than what I've said, caps look cool and get some wow-points from people who're easily impressed.
