It really is down to personal preference of the song list, but that being said there is absolutely tons of DLC available for both with many songs being offered on both GH and RB. I personally prefer RockBand as a multiplayer game, but on guitar alone I prefer GH - but I own and play both.
I might even recommend you just go ahead and get the whole kit with drums, guitar etc... This is the type of game that even non-video gamers tend to enjoy. We occasionally have family RockBand nights where even my technically challenged mother-in-law will grace us with some terrible vocals while the rest of us try and outscore each other on guitars.
Also, RB3 was just released though I haven't tried that one yet.
Good luck with your
son's purchase!
