Personally I find FM transmitters to frequently have poor quality and often need changing of stations to stay clear. Given the option I would definitely pick the cassette adapter. Less chance of interference.
Cassette adapter here http://www.mp4direct.org/Other%20MP3%20Players%20&%20Accessories.html this one
BELKIN Mobile Cassette Adapter for iPod Model F8V366-APL
$12.99
BELKIN Mobile Cassette Adapter for iPod Model F8V366-APL