Id go with 1 15. While the two 12's may have more output, its more badass to have one really loud sub playin. And it will go much deeper, and bass is all about hittin low notes so Im a big 15" fan. 12's are too middle of the road for me. If you want great SQ go for 8's or 10's, but if you want real solid knock your ass off bass, go for the 15. 12's sound good and get low alright, but does not excell in either category as much as some other sizes can. This however, is all dependant on the install. But if you do it right, id say go for one 15 ported to 32 HZ