a 15 has a deeper sound and takes up more room, pushes more air, and is louder. While a 12" IS BETTER FOR ROCK but requires less space and will still be loud. however the 15 will probably rattle your car more because it pushes out bigger sound waves.
The type of box you put it in will matter just as much as the subs.
Now for sub size I can only give my opinion.
15" will hit deeper notes such as a kick bass good for rock.
the 15 will also hit lower notes in general compared with the 12"
But this also relies heavily on the box and amp you use.
The 12" will still hit pretty deep but still heavily relies on the box and amp you use.
I prefer 12" subs myself for general music listening but I think its all up to preference.
