No, there shouldn't be a difference in sound between the different impedance subs. However, the higher impedance ones (like 6 or 8 ohm) will be easier for your amplifier to drive and would probably be the best bet. Generally, a sub is a sub, though many car subs don't concentrate on sound quality and are much more about high output. A good car sub (like a JL Audio W7) sounds great no matter where it is. However, if you wanna try to be safe, a good HT sub will be a great bet. Another option is the Dayton Titantic 15" driver (or sub kit) from Parts Express. I can't post a link, but the url is just Parts Express with a .com on the end. Search for "Titanic Sub" or just look in the speaker building area.
The part number for the 15" sub driver is 295-420. The kit is 300-764. Happy hunting!