Not going to get into a "my gaming system penis is bigger than yours" contest.
I have played both RB2 and GH-WT for PS3. If I had never played RB I would be blown away by GH. But I have. GH pales in comparison.
The setlist for GH is interesting. Beat It and No Sleep til Brooklyn for instance, offer some different choices. There's not nearly as much fruity crap you've never heard of as there is on RB2. But when it comes to downloadable content, there's absolutely no comparison. RB blows GH completely out of the water. And the backward compatibility is sweet. Merging the songs from RB1 into RB2 is a really cool feature.
GH is simpler to beat. One fault I have with RB is that the difficulty level between medium and hard increases exponentially. You should not be able to crush a song at 98% accuracy on medium and then struggle through 16% of that same song before failing out on hard. But that happens on RB. The increase in difficulty in GH is more manageable. If you're hitting 98% on medium, you can expect to pretty easily nail 75 - 90% on hard in most cases.
But in terms of overall experience, graphics, gameplay, musical choice and "fun" quotient, RB is the standard by which all else should be judged.
GH -2.5 stars
RB - 5 stars