As a rule, smaller subs are better for punchier bass, but they will NOT go as alow as a 12 or 15. So 4 8" subs will get loud, but only to a certain frequency. Don't expect to run some bass mechanik CD running at like 10hz to sound that good on an 8", even 4 of them..
It all depends on what music you like and what kind of bass you're looking for. If you like SQ and listen to classical or Jazz or tight bass drums in most rock, then the 8s will sound fine. If you like boomy, superlow bass from rap and such, then you'd be better off with the 2 12's or 3 10s.
Since you're probably going to be using as much trunk space with 4 8's as the 2 12's, just go w/ the 12s. IMO