Hyatt Diamonds do not usually receive complimentary suite upgrades, so in that department SPG has an advantage (at least for hotels that actually follow SPG's upgrade policies, many don't seem to).

I would echo what others have said: Hyatt Diamond treatment is superior to SPG Platinum (whether 50 or Ambassador). It seems that Hyatt has a tighter grip on the execution of elite benfits compared to SPG where upgrades etc. are not consistently offered, at least in my experience. Further, SPG has a number of properties, such as the 4 Points in Miami, that are in deplorable condition. I have not encountered similar properties at Hyatt or Hilton.

I am keeping SPG Platinum at the 50 night level as my backup program due to the value of SNAs. I don't see much value in the 75 and 100 levels. The ambassador service in particular was never executed well for me and I subsequently dropped back down to 50 nights.