Of course what SPG does is make most rooms something besides Standard so they can't be booked.

Honestly I find booking rooms on SPG harder then on Marriott where I rarely have issues.

I just checked both chains for a weekend in London. SPG came back with one option. Marriott came back with about 10.

I also don't think the "once the limited rooms are gone, that's it" statement is 100% correct. I have been watching for what I wanted in Paris for while and "bingo" a few weeks ago the hotel opened up again after having been out of award rooms for a month or two.