That's a tough call.
I have stayed at both and both were nice.
The JW is newer and IMHO somewhat nicer, but more isolated.
The Hyatt's location is closer to Disney.
I also think that parking was free at the Hyatt and I know they charge at the JW. (I may be wrong on Hyatt..it's been a while)
My personal fav is the JW but the Hyatt is not a bad option!
Neither is a bad choice.