I have had experience with both the FM adaptor and the cassette adaptor from trying to run a portable CD player in the car. I had much more underlying hiss from the cassette adaptor than from the FM adaptor - possibly partly due to the quality of the cassette player, but I think hiss is more of an issue with cassette players than FM tuners in general. If you generally crank the volume up then maybe the hiss won't be much of an issue.

The FM adapter I use plugs into the cig lighter and has four voltage settings and different power plugs for powering the CD player (or whatever other tunes device you're using). It only had a couple of frequency settings but I never had a problem with that.