I have played RB and seen plenty of videos of the game on youtube. I was unable to pass Green Grass and High tides due to differences between the soundtrack on the game and the one I have on my iPod, but my comment is mainly in reference to the charting of the solo in Battery. Despite the smaller timing window, I find it much easier in RB than GH because the patterns are much simpler. Also, the difference in timing window creates some good and bad things about the gameplay. The notes are easier to hit in GH, but due to that, the top scores can only be obtained by squeezing (manipulating the timing window to get more notes under star power). I have not had very much experience on RB but the DLC looks good, especially the Iron Maiden pack.
I think RB definitely offers the best experience for drums unless you are a Metallica fan.
Most evident in GH3 is the much larger timing window and the weird setup for hammer ons. They fixed it a bit for the GHWT interface but it's still larger than any game Harmonix ever made (including GH1 and GH2).
TOURNAMENT
I am making a tournament for GHM (or GHSH) and GH5, plus RB2 if people request it. The tournament will be open from October 10 until October 26 2009. I do not care if the mods make it official or not because the point is to have fun and not to win honors/awards.
Please suggest songs if you plan on participating.

lol