I dunno, on the FM transmitters I kinda feel like you get what you paid for. The monster brand one has been good for me. No intereference hardly at all from other stations (this from a radio saturated place like Vegas). About the only time I change channels with mine is when i'm travelling to another city and there happens to be a station on the one i'm using. But switching is easy, and it's not often that of my 3 stations I can't get a clear signal on at least one.
That said, I haven't tried the aux input....bu if it's as easy/cheap as people say I may give it a shot.