that is the question of the day.
i have 1200w RMS @1ohm to feed either combo.
right now i'm sitting with 2 12" L5's in my spare room but i've been partially persuaded by the recent install by Canadian GP and his son of their XXX 15" monster.

anyone?

Well for spl, of the same drivers, 2 12's would be slightly louder than 1 15.
Other than that, I would say go with 1 15.
Easier, smaller enclosure space, and only one driver.

Two IDENTICAL 12's have the potential to get louder (more cone area).
But compare 12" L5's against a single massive 15" and things change considerably.
Just choose the single 15" that suits your needs, power, and box area and you won't be dissappointed.
I am staying with a single 15 but I need one that can handle 2500rms+ daily...

Plus the aww of a single sub, less concellation, yada yada...

The single 15" generally makes more sense. Cheaper (in this instance it's not so much an issue), lower Fs, smaller enclosure, less weight (again, in this instance this fact is not as prevalent). The XXX will sound sonically superior to the L5's within reasonable box sizes (ported or sealed - L5's have the capacity to sound superb, however, they require an inordinately low Qtc which translates into an ungodly sized enclosure). Go with RE.

^^ I'm not sure he asked in general about the xxx, just about a 15, which is why I said "of the same drives". Meaning 2 12 l5's would be louder than 1 15 L5..etc.

Ah, my mistake. I read that too quickly. At any rate, my point is still valid. Except there's an added benefit to the single 15 (of the same brand as the 12) in power required (less), which equates to less money.

One big-ass 15 is better than two 12's imo. No audio theory reason, just cool factor...