In a one word answer, NEITHER. I have a FM modulator for my system, and it sounds like crap. The sound quality rivals that of an 8-track player... ewww... I don't know about an FM transmitter, but I'm pretty sure they sound just as bad if not worse. If you're still interested, this is the exact model I have: http://cgi.ebay.com/aw-cgi/eBayISAPI...tem=1220641520 . In my opinion, I suggest going out and buying a nice cd-player with auxillary inputs on the back. If you can't find one in your price range, then try for one with cd-changer inputs. For most head units, you can get a dongle to plug into the cd-changer port to create inputs. But that's just my 2 pennies.