imo the larger the speaker, the greater the amount of air it pushes, giving a more body punishing wave.
ofc theres a lot more to it that just that, but if youre having a custom box made for the PSI 12", its basically the same as calling it a 'port setup'.
depending on the performance you want to get out of the sub speaker, you have to look at its response range ... can it go as low as 5Hz (rare but subs can do this)? whats its bottom end response? can the response be altered (some subs can do this)? tailoring your sound results in the best performance (to your ears) and so i would always suggest getting a sub with the widest response range (to please the majority of ears).