I figure I would start this thread since HR recently opened and some have had this dilemma. This is my 2 cents. I am a Globalist and this was my first time in Australia. I spent 4 nts at HR, went to Cairns x 3nts, then 2 nts at PH. I did plan my itinerary based on where I was staying (eg, I planned trips needing ferries for when I was at PH.)

Hyatt Regency: cash, 20K points, or 125 USD+ 10K pts
-in CBD
-walking distance to a lot of activities/eateries
-direct access to the Merlin pass attractions (Aquarium, Wildlife Zoo, Madame Tussauds)
-lounge for breakfast and evening canapes
-No pool

Park Hyatt: cash, 30K points, or 300 USD+ 15K pts
-Aspirational Hyatt property. Arguably the best hotel in Sydney.
-breakfast with Opera House view
-not guarantee to have a room with view unless you pay for it
-has a small pool with nice seating/cabanas and indirect Opera House view
-Circular Quay- major hub of public transportation: ferries, subway, buses

Value: unlike HR vs PH in Tokyo and Paris where the pt difference is 12K vs 30K, there is less value here with a 20K vs 30K difference. 10K to upgrade from HR to PH is a steal. The value comes cash (HR can often be 1/4 the price of PH.)

Conclusion:
If traveling alone or with a partner, I would stay at PH. However, after hearing all the rave reviews, I personal did not find PH Sydney as impressive as Andaz Tokyo, PH Vendome, or PH Vienna. (I am also not as impressed as others on Flyertalk of PH Tokyo or PH Milan.)
Traveling with a family, I found staying at HR to be a little easier. I like the access to the Merlin pass site, having a small bite in the evening in the lounge when the kids were too exhausted to leave the hotel, and having so much within walking distance while staying at HR.