If you don't stay in the hotel much except for sleeping and going to the RC for breakfasts and snacks, go with Bali Hyatt as there are tons more things to do in Bali.

If you plan to stay in the resort (room/pool/gym) then go with Hua Hin (RC rooms are great), though as SFO_FT said, the town is not great (never got to the beach while we were there) and it was cloudy the whole 3 days we were there.

Both will most likely upgrade you to RC even if you book regular award rooms (insiders @ Bali Hyatt told me that this is automatic there). If you plan on going to the hotel spa, Bali Hyatt beats Hua Hin hands down (quality-wise), though the prices at the Bali Hyatt spa is a little outrageous IMO, and the spa in Hua Hin is pretty decent.