It depends what kind of sound you want from the Sub. The Amp you have will easily power either of these subs, so the only difference will actually come from the sub itself and will have nothing to do with the Amp.
The 15" is very loud, and plays very deep base (makes the hairs on your head vibrate, as well as everything else
) as well as your normal bass levels, where as the 12" will play tighter bass lines.. so... its up to you