^I can definitely agree with that. I love the way new challenges appear depending on what DLC you've bought, for example.
It's a matter of taste, but I don't agree that the music itself is better in Rock Band. I've found some of their choices distinctly lacking. The representation of that music - the note charts, especially when the same song is available in both games, is generallly better though.
It's perhaps due to their desire to appeal to a wider audience, but Rock Band's definition of "rock" is often highly dubious.