Ho-kay so, I have played both version of GH II and have played GH I, if you havent played GH II for PS2 you might not want to play it on the 360. Regardless, gameplay is virtually the same. The guitar however is not, the keys are closer together and easier to hit which is nice, but the guitar itself is a bit goofy and can be a pain in the ass....well arm. The guitar is considerably smaller for the 360 than it is for PS2, and it is shaped weird. The new songs for the 360 are fun, one of my preferences is "Dead!" by My Chemical Romance, not a very hard song after you get the rhythm down, but fun regardless.
Graphically there is a bit of a difference, although it is not huge. The characters are a bit clearer with more detail and the rock meter is a bit more vibrant. Overall a good game.