I own at Mission Hills and, while I agree with Dean that that you cannot compare a Palm Springs to a California coastal resort (especially Four Seasons) and with everyone that I would seriously rethink whether to go to the desert in the summer, I would strongly disagree with the assertion that the Palm Springs Marriotts are more highly rated or regarded than Westin Mission Hills.
I visited both and bought at the Westin because the 2BR units at WMH blew away anything that Marriott has in the area. Desert Springs is a beautiful resort with superior landscaping and interesting canals, but I will take the units, pools and facilities at WMH over Marriott any day. You will see a lot of talk about the Marriott units simply because of the number of units at Desert Springs I and II and Shadow Ridge create the most noise. As always, take a look at the TUG reviews and decide for yourself.