Personally I wouldnt go for the radio transmitter, it may be the cheap option and when you start using it you find out why. One of the main probs i had was i could tune it in fine on the driveway at home but by the time i had got to work 5 miles away another station was competing with the frequency i had set, aswell as the interferance on the journey too.
Looking at getting a proper iPod adapter (Pioneer CD-IB100) now for my Pioneer AVIC-X1 HU, did look at the dension myself when i had my standard HU but having a 2005 Clio, Renault have changed the HU and there werent any expansion ports available.
Chris