Well...
I own GH2, GH3, GHWT, GH: Metallica, and GH: Smash Hits.
I also own RB1, RB2, RB: AC/DC, and over 2 GB of RB DLC.
And my conclusions:
1. I generally prefer the way GH displays notation for guitar. That is, I prefer the rounded notes to the square notes. On drums, the reverse applies. Actually, I'm kinda undecided on drums, either way works for me just fine.
2. I prefer the 6-piece drum play in GH, although I think that, overall, RB has more interesting note charts.
3. When it comes to DLC, Rock Band absolutely thrashes Guitar Hero. It's no contest whatsoever. Plus, you can import the entire RB1 setlist (minus 4 songs) into RB2. That almost doubles RB2's content.
4. The GH controllers absolutely thrash the RB controllers.
5. The Hyperspeed mode in GH is better than the analogous Breakneck mode in RB.
I end up playing RB much, much more than GH. Ultimately, it just has more music that I like. The Iron Maiden DLC doesn't hurt.
