The new guitar is heavier and bigger. The buttons seem harder to push in than the GH3 guitar and the strum bar is "clickier", as it makes far louder clicks and is harder to strum quickly with. The + and - buttons are difficult to press quickly as they are very small but the new star power button is a nice addition. The whammy bar has also improved.
The touch pad is not very responsive and the parts where you use it can be replaced with easier button presses.
So if you have a GH3 guitar, unless you want 2 guitars for multiplayer, I don't suggest you get the new one.