AS for the sub you should get, remember that each brand of sub sounds different, and that its also affected by the box's size and whether its ported or not!! I'd highly suggest going to your local car audio shop & testing out one of their systems that closely match what you are looking for to see how it sounds.
I think that you're right about a 12" drowning out alot of the sound from rock/metal music. I have a Rockford-Fosgate DVC sub w/ a 600w Amp, and yeah, its boomie as hell, it sounds great with techno/rap/hip hop ... and it sounds good with rock, but I think a pair of 10's, maybe even 8's would sound better. 8's would bring together the sound in the car ... 10's would give you a bit of the boom, be MUCH more responsive overall than 12's, you wouldn't be able to hear them outside of the car much because they are 10's, unles you bought high-end ones. I have seen some sweet 10's in my day, they are out there, but the best thing to do is to test some subs with the type of music you like ... and TEST the subs, give 'em a run for their money at the store! I think that the only thing you'll lose out on with 10's instead of 12's is a bit of the boomy-ness ... 10's would be better & much more responsive in my opinion.
Good luck, peace out!