The RC is posting rack rates on the order of double all the other luxury hotels in Tokyo and the initial reviews are strongly suggesting that it isn't worth it. There is a thread in the luxury hotel forum about this hotel.
There is no comparison between the Park Hyatt and the Sheraton Miyako. IMO, there is no comparison even between the PH and the Westin. The only appropriate comparison currently would be between the PH and the FS Maronouchi.