GH4 is much more advanced than Rock Band and looks to be even better than Rock Band 2 with how quickly that game is being forced out.
It will have more songs, a better create-a-rocker mode, a sturdy and more efficient drum kit, a better guitar with a touch sensitive strip for tapping/sliding/improvisation (not like it's difficult to have a better controller than the horrible RB Strat), and best of all the ability to create or recreate any song you want which you can upload for other people all over the world to play.
So until we see some huge improvement with Rock Band 2, GH4 is looking like the better buy.
As for U2 having songs in it. I don't think the simplicity of their music would keep them out of the game at all because there have been many songs with easy riffs and solos before in both GH and RB. Most of their stuff would just probably be in the early to middle teirs in the setlist. Also, since now the games take each part of the music into consideration, an easy guitar part doesn't matter as much if there is a difficult drum or bass or vocal part. So U2 songs would definitely be good additions depending on which ones they are.