Whenever I play Rock Band, I always use my 360 Guitar Hero 2 controller. Reasons include a functional strum bar, a solid design, and consistent functionality. That's a nice thing about the Rock Band/Guitar Hero divide, for as long as they undercut each other, those mics, drums and guitars are going to work together, by and large.
Of course, to be fair, the two DJ games currently announced, DJ Hero and Scratch: The Ultimate DJ, are very different products, with very little shared between the two. It would be nice to not have to buy a whole new slew of peripherals, however, which has the potential to limit sales for both games.