Ipod Connector

To be honest, FM transmitters are the WORST possible option for listening to your iPod in your vehicle. The digital>analog conversion is usually bad. Even in a best case scenario, the sound will only be as clear as FM radio, which is pretty bad and poorly compressed.

Even if you don't care about audio quality, there's the element of interference over the airwaves, which can render FM transmitters useless in large cities with lots of broadcasting stations. I haven't encountered an FM transmitter that will work anywhere near the city of San Francisco and even had a hard time locating a clear frequency during a cross-country road trip.

If, after this, you're still convinced the convenience of FM transmitters outweighs the cons, then you'll probably want to look for a unit that offers automatic tuning (such as the Griffin iTrip AutoPilot with Smartscan) that will at least save you the trouble of searching for a clear station.

The radio transmitter is OK if you normally travel within a small radius where you don't loose stations. As stations fade in and out you will need to tune to other unused frequencies to eliminate interference.

I believe a better, less expensive option is the cassette deck connection. If your audio system has a cassette player you can pick up a $10 option that will connect almost any audio device to your car stereo. This is a good solution for older car stereos. Neither is perfect.