You should check out the song lists for yourself to see which games suit your tastes better. I personally like Rock Band, but personal preferences aren't much to go on.

Also, note that both systems let you buy individual add-on discs, with songs from particular genres and/or artists (Like Aerosmith or Metallica, etc.). Also, I know that Rock Band will let you buy individual songs from an on-line catalogue for about $4 a song.