I think Rock Band 2 and GH:WT will be equally good. Some will prefer RB, some will prefer GH:WT. You should listen to all the songs in the setlists of both games and then decide which one you like best.

Personally I prefer GH:WT's setlist (about 55 songs have been confirmed as of yet), also the new guitar looks better than RB's guitar, so I'm probably getting Guitar Hero. Plus Rock Band 2 won't come out in Europe until christmas or something, so I'm just getting the whole GH package and maybe sell my drums when RB2 comes out and buy that drumkit plus 2 cimbals.