GH5 is a much more polished game. The Party Mode is brilliant - such an incredibly laid back way of playing. I'm only a 'Medium' Guitarist, but I find GH5 a tougher game.
Songs - nothing between them for me, though once the Store is open you can use your DLC from WT on GH5 (and I think the WT songs will be available later).
As I said, GH5 is a much more solid game as a whole.