It looks the same, but it's not! On the inside, anyway.
The touch strip on the neck used to be analogue but now it's completely digital, so Activision says it's now "100 percent accurate." The strum bar has also been redesigned internally, so it's way more durable. I didn't actually get to hit it to see if you can feel how much more solid it is. But hopefully the DJ Hero turntable is a pretty nice indication of the quality of hardware we should expect this generation. The whammy bar has been subtly redesigned as well, and now you've got some fake chrome tuners instead of fake plastic tuners.
I've always preferred the Guitar Hero guitars to Rock Band's since I think the latter's strum bar is too squishy. What were you hoping would change with the new guitars?