I'd recommend that you get 2 12"s of whatever brand you go with. Its so much easier to make a pair of subwoofers louder than a single sub.
Get a pair of RL-P 12". They will suit your needs nicely with 1000W vented @ ~30Hz.
Unless you have a hell of a lot of power, your not going to make a 15" louder than 2 12". And if anybody talks a bunch of **** to me for saying that...they can **** me off because I've done the test...and I know what I'm talking about.