RB2 without a doubt.

The RB2 interface is better, more focus (GH try to follow, as you can see with the progress bar addition in Metallica).
The RB2 sound is better than the WT sound (Metallica however catch up).
The RB2 solo tour is more interesting, very lenghtly and allowing you to include DL/RB1 songs in concerts, giving you specific challenges.
The RB2 group gameplay is the just way above GH. RB create a true synergy : you can help/Save your mate, need to synchronize your starpower use, but still have solo to show off, you can blow off the great Final if one fail, you get nice titles at the end like "savior of the group" and stuff that put your role in light. GH just put 4 guys next to each other, barely sharing the starpower, that don't really need to watch over what the others are doing.
RB2 is fully compatible with RB1 and all DL songs (you can merge all that). WT is not comptabile with previous GH, neither Smash Hits nor Metallica. each time you want a song, you need to swap discs (GH5 will be however, with WT only).

Overally, Neversoft is doing a good job, but they're new to the genre (from Tony to GH...) and you can see they work mostly with what's Harmonix left to them. They've got pretty good instruments (thanks to Red Octane) tough.
Harmonix have been doing musical games since 1995 and they're currently the best at it.

It really depends on how serious you are about drums. If so, get World Tour, because I feel Guitar Hero nailed drums much better than Rock Band did, including the ability to do a double pedal in GH: Metallica and GH: Smash Hits for an Expert + difficulty. Otherwise get Rock Band because there is far more content, and Rock Band 2 frankly has a better tracklist.