I've stayed at both and they are both quite nice IMHO - just different!
The Bali Hyatt is older and was remodeled recently and you have the Sanur situation, which I like. Nice Regency Club. They used to give a FREE massage per day in the RC.
The Grand Hyatt Bali in Nusa Duah is newer (about 10 yrs old) and has many fine qualities including views, a nice water slide and larger grounds- but is somewhat isolated and you lose much of the magic of Bali at places like this!
It is a very nice place and I did enjoy it!
In short, I'd go back to either!
Anytime!