Guitar Hero: World Tour and Rock Band II are pretty much similar. GH:WT will allow you to create your songs but with heavy restrictions (I assume due to licensing stuff). We play RB2 because it's well established and there's already a sizable catalog of songs, either through DLC or from importing the old RB1 songs into RB2. RB2 also has 20 free songs coming up this fall and numerous full album releases planned (Foo Fighters, No Doubt, Megadeth, RHCP, Rush, etc.). There's also weekly releases (Bad Religion and others this week, for example). GH:WT will obviously have DLC but you'll be starting from scratch.
Check out both playlists and pick what you feel would be best for you and your friends, frankly. It's really about the fun you'll be having, game mechanics are pretty irrelevant for these types of game.