Ive heard from many people that if you want a better range of tones and clarity, 10's are better than 12's. ive also heard proof of this myself. all my friends go out and buy 12's because they want people to hear them from blocks away. problem is, they sound like sh1t. outside the car, you hear nothing but rattling(due to things like license plates and trunks) and thumps. inside, you hear only the bass and the rest of the music is drowned out. yeah, some of this is because they are idiots and turn the treble way down and bass way up. ive heard 10's in well built boxes and good amps that can bump just as well as 12's. my ignorant friends seem to think this is not possible. they are stuck thinking bigger always means better. who else agrees with me that some good 10's sound a lot better than 12's anyday.(and i mean for overall tone and clarity, not just bass)
generally speaking, 10" subs will be faster due to the lower driver mass, so they can sound tighter, and mroe snappy, but as a trade off they don't have as low of an Fs, and thus, don't offer the resonance and lower frequency extension that a twelve will have. Personally I find fifteens and larger to be slow, and sloppy, and tens to lack the low end extension I prefer. I find 12" and 13" subs to be an ideal balance between low end response, and tightness and speed.
I don't really care how the system sounds outside of the car to be honest. The more you hear outside, the less you hear in the car. Wasted energy. Ideally you should hardly hear anything outside of the car with a good system, regardless of the SPL obtained inside the cabin.