you all seem to be (except for integraz) under the misconception that the bigger a sub, the "laggier" it is... not flaming anyone here but thats an utter load of crap, it all comes down to how well the box is designed and constructed, yeh a 12 and 15 will generally go lower in the frequency range, thats cos its bigger and can move more air, but i beg someone here to go and find me a 10" "subwoofer" that cant play below 28hz, anything below that is probably in some pipe organ music and i doubt anyone here listens to that :P

so to the op- go the 12, its a good medium, it will have plenty of out put given a decent amount of power and i will probably give you the most enjoyment