If the 3515s are a good price, get them. It would be stupid to get something that cant take much more power than you have. If you want to upgrade later with the 2515s, you have to upgrade subs and amp. With the 3515s, you could just get a second AQ2200 down the road and youre set. The 3515s will be just as loud if not louder than the 2515s on the same power, whether that power be 500 RMS or 2500 RMS.