Both games are superb, but I've never owned or completed (as in 100%) Super Mario Sunshine. I've been dying to, though, but the only person I know who has it and could lend it to me is a jerk and I don't have the money to be spending on video games at the moment. The FLUDD-less levels in Sunshine were just fantastic, as well as the hub world and the music, etc.
Galaxy on the other hand is the longer game, and anyone who thinks it's devoid of any difficulty obviously didn't complete it twice. Its hub world, while slightly barren, was still pretty charming, plus as Feenie said it had some nice eye-candy in the form of Rosalina. Of course it goes without saying that Galaxy also has superb music, and a less finicky camera. I can't wait to play Galaxy 2...
@Corbie: If you're not going to play Sunshine anymore, how about passing it on to someone who's been dying to give it another play-through for a while? I'd give it a good home... 


