I think one other thing to consider is the types of properties you're looking to redeem your rewards at. I find that despite Marriott having so many more properties, they don't have as many "high end" places. I.e- JW Marriotts.

Quite often, a Westin or Sheraton at Starwood will be in a very very nice place and that's ignoring the really high end properties like st. regises etc. In general, I rarely feel like a full service Marriott is "special", but even westins in good areas will stand out to me.

Maybe it's just personal preference or where I end up traveling, but that really separates the two programs for me. If you do a lot of travel to mid-range cities and want lots of travel options for cat 3-4 hotels, then Marriott probably will have a superior selection. If you are doing cat 5-7 places then SPG gets the nod in my book especially with the treatment you potentially get as a platinum.