Mike is absolutely correct obout the differences in the 10's and 12's.
What kind of car do you drive? If it is an F-body, is it important for you to keep your t-top storage? If so, you can have one ten in the back left corner of the rear hatch or 2-10's on the flat spot right behind the rear seat. If t-top storage is not important, you can do 2-10's, 3-10's or 2-12's in the rear cargo area with out any problems.
If your car is a hatchback, it is going to get very loud. Hatchbacks tend to get anywhere from 3-6 db louder than a car with a trunk (same amps and subs).
IMHO, I personally like the 10's and alot of power. I am a firm believer of buying as much power as you can afford.