I would recommend getting a PS2 for the simple reason that you can't go wrong with that purchase... period! :) You can play PS1 goodies too!!! :D
Since you already have a 360 then I'd recommend against getting a PS3 (pick it up when it's cheap), however, if there are tons of exclusives you want NOW then go for it.
I wouldn't get a PS3 for multiplat games because regardless which version of game X is better, in most cases the differences are unnoticeable unless you nitpick.