well as far has high revs, the shorter the stroke, usually the faster and higher the rev, hence why as a general rule smaller capicity motors rev harder and faster, thought not always the rule. Acording to that theory and a12 will rev a little harder than an a14/15. Their highest revs are about the same,(6000rp,) but the a12 would probally get there faster. (the shorter stroke)

power, you mean torque, then an a15/14 is the go.
An a12 can go just as well as an a14/15, but you have to spend a little more money on it to do so. (its about 10hp less in standard form i think, 81hp to 72 from memory).

But bigger, better, faster more. is a simple rule that will always bring good results.