I have stayed at both Hyatt hotels in Bali.
Grand Hyatt (nusa dua) in 2003 and then Bali Hyatt (sanur) in 2008, 2009 and 2010.
So no prizes for guessing which Hyatt is my preferance.
Bali Hyatt, although a much older hotel, it is more traditional balinise style and the hotel grounds are magnificent. Excellent service by the attentative staff and if you pay the extra $ and get a Regency room you recive additional benefits including discounts for some of the hotel services.
IMOH you will not go wrong with either hotel.