Sure, Guitar Hero: Warriors of Rock hits first but there are plenty of loyal fans for both huge franchises.
Rock Band 3 is due out in October and a recent RockBandAide interview with Harmonix's John Drake should be of interest to ardent followers. Feel free to listen to the entire interview if you wish; the most important tidbit is that RB3 will allow you to export RB2 tracks for free. This was expected, as RB2 let you do this in regards to the original Rock Band but still, it's good info to have. Then there's something else to take from that talk: the now-standard price for extra songs via downloadable content is $1.99. However, Drake stated that any songs for Rock Band 3 's new Pro Mode will likely be more expensive than that, although he wouldn't provide a number. He admits that Harmonix has plenty more work to do on those tracks, and they don't intend to increase the cost of the regular tracks, which should remain at $2 apiece. If you aren't familiar with Pro Mode in RB3, let's just say that it'll require some definite dedication and patience.
The new Guitar Hero may invariably win the sales war but it's highly probable that Rock Band 3 will perform well on the charts.
Related Game(s): Rock Band 3