My thought, for what it's worth:

In the Starwood system, you ain't squat unless you're Plat. The Gold just doesn't mean much in the way of upgrades, etc., although it does give you that extra point per buck. Platinum, on the other hand, gets you upgrades to suites and access to Concierge Rooms. So, Platinum Starwood is valuable. (But you already have that, so . . .)

In the Marriott system, Platinum doesn't really do much more for you than Gold does. You get room upgrades, Concierge access, etc., with Gold. Oh, you get an extra 5% bonus on your base Marriott Rewards points, but the gap between Gold and Plat in Marriott is not the gulf it is with Starwood. So, not worth burning your upgrade for MR Plat unless nothing better comes along.

(By the way, you indicate you're 11 nights short of earning Plat. Do you already have the Premier MR Visa card that gives you 20 bonus nights the first year, and 15 each year thereafter? If not, that might be your solution.)

Between Hyatt and Hilton, I'd go with Hilton Diamond. Again, Hilton does have more properties worldwide (but what really counts is the area where you travel, so check that part out). If Hyatt doesn't have more properties in your travelling area, I'd go with Hilton Diamond. The Diamond gives you lots of perqs on upgrades, "forcing" reservations, etc.

In sum, go for the Hilton Diamond, and check out the quick boost from Marriott Gold to Marriott Platinum through the credit card, but even if you aren't eligible for the card or already have those nights calculated into the 64 nights, your Gold is nearly the equivalent of the Platinum. And of course, hold onto the Starwood Platinum; if you have enough stays to earn it, it gives you good value in return.