Most peoples Idea of the perfect car audio system is >100db inside the car and 0Db outside. The more sound you let out of your car the less you'll have inside.
I see lots of people driving around with their bigass stereos pumping out tunes to everyone within a five block radius. This is actually undesireable. If people continue to -play their systems so that everyone outside can hear them, noise restrictions will become tougher and the car audio market will lose alot.
My recommendation is go with what sounds good INSIDE the car. Then line the car with dynamat or the like, to keep all your sound in the car