Depends on who you ask. Both come with the ability to DL new tracks every few weeks, and both sets of "instruments" work with each game, so it's really down to which you like the look of, or the music on the original discs.
GH:WT does have cymbals for their drums, while RB2 you have purchase them separately. GH:WT also has the ability to create your own music and upload it for others to hear. I would think that unless you are a studio geek, this feature wouldn't be used but once or twice. Don't know though, never used it.