I pretty much completely agree with what Kopstudent said. I think SMG2 fixed pretty much every complaint I had with the original Mario Galaxy. It's just the first game was such a beautiful revelation for me, that I think there was very little SMG2 could've done in the form that it was to capture that again.
I was a big fan of the proper overworld hub from SMG1 though, I was quite sad to see that go.
I think if I ever replayed them again it would just be as a pair anyway.