i gotta extended cab silverado and i want some subs but i was wondering how 1 15 compared to 2 12s of the same model would sound because i am just going to set the box in the back seat and two 12s take up too much space
Two decent 12's would be louder. If you think about it, one 15" sub would have to have a lot higher excursion than to 2 12's in order to move the same amount of air. Two 12" circles will have more area than one 15" circle.
However, if the music you listen to has a lot low, deep bass. The 15" will be able to produce those notes with less distortion, but 15's won't produce the higher notes and punchy beats of hard rock songs very well. I prefer 12's myself, they are more versatile and are able to produce tight, punchy bass in rock music and they do a pretty good job with the low bass of rap.
It just depends on what kind of sound you like and what kind of music you listen too.