Easy: Rock Band. Four different instruments to play (drums rock my anus), the song list I personally feel is much more refined than the Guitar Hero series (and downloadable song packs are always great), generally more my genre of music can be found on Rock Band compared to the GH series (although Madhouse for GH2 reminds me well of my great upbringing \m/.)
Also, one big point with me in choosing Rock Band over GH is you can create your own character. I like creating people. It's a perfectionist hobby of mine.
A big setback with me for GH3 is that, it seems damn near all of the note charts for the higher difficulties have notes thrown all over the damn place to make it harder for the GH wankers. When I play a song, I actually want to feel like I'm
playing the song.. note for note, or at least as close as a video game lets you. GH3 didn't do that for me at all.