fart said:
both sound like crap.
for the record, i use the tape adapter.
I used a cassette adapter at first, but the sound quality was horrible and in cold weather, the adapter's spindles would stiffen up and the car's tape player would think it had reached the end of a side and flip to "side B", which didn't work. So I got an iTrip and have been pretty happy with it. YMMV of course -- it varies wildly depending on where you are, how crowded the frequencies are, and what kind of car you're driving (since the antenna is usually in different places). It works REALLY well in my convertible, because the antenna's in the windshield.
I like the idea of the units that have their own full-frequency tuners built-in, like the TuneCast 2 or that other one that includes a dock. And it's always preferable to use the line out built into the dock connector -- you'll get better sound quality than anything that connects to the headphone jack (like an iPod).
There are a number of other options. The
IceLink connects to standard CD changer connectors that are built-in to many cars, and even allows some control over track selection from your head unit. Or you can get Alpine's
KCA-420i, which interfaces with many of Alpine's head units and gives you much more control over the iPod through the head unit, even displaying playlists and track information. Alpine's having a sale on the unit right now.