i just beat gun on the 360, I had played about half way through on the xbox. The 360 version does look better (if you have a HD) i put it on the xbox and the 360 at the same spot and at the same time and compared...
gun on the 360 does not look even close to how good a 360 game can look, but it is way smoother and clearer at 720p than the xbox at 480p. the textures are slightly higher res.
would have been nice if it was done more so to take advantage of the 360 but my opion is that as it was developed for the last generation systems it would have been too much of pain to redo all the models and textures in a true high res format.
looks like a very very clean xbox game... not a 360 game IMHO
still a fun game though