My favorite excuse to show off to my friends.
Guitar Hero obviously focusing on guitar riffs and insane finger breaking songs was more of a single player experience. Yeah, okay you can play with a friend or go online to play 1 on 1 but that's beside the point.
Rock Band on the other hand isn't really about the guitar (even though when at my friend's house that's all I play anyway) it's more about a band feeling. It's a party game that combines the coolness of guitar hero with the embarassment of playing Singstar and the craziness of playing the drums.
Now the game launched with I believe 54 songs while Guitar Hero 3 launched with 72. However a pack of song titles is now being released about every week for Rock Band and I don't think there is a schedule for Guitar Hero expansions. By now Rock Band has plenty more songs than Guitar Hero (although you have to pay for them but same for Guitar Hero)
Although Rock Band has proven to me to be a better party game and music game it is still hard to have real fun on it unless you're house is a cross roads for everyone in your town. The online play isn't too fun because you can't chat while you're playing (plus you can't hear the singer make a complete arse of themself over the mic) Online would have picked itself up a considerable sum if it had included a kind of online world tour which is the new fresh game mode that it pulled out with over Guitar Hero.
Again even with the two new instruments added (singing with me and my friends isn't a good choice because the first thing you say will be a sign of weakness and you'll get hit in the back with a guitar or drumstick) the one I play anyway is the guitar. I shred with guitar I can play every song in that game on expert with the guitar behind my back (yeah it does ache) I love Rock Band because now I can gloat for every glorious victory I have right in my friend's face.
Rock Band is a great game and if you have the extra cash you should buy it over guitar hero. Also if you're a gloating jerk like me you can't go wrong with this. If only they made a Polka Band then i could really show off my Accordion skills.