I tried Marriot "only" for almost 20 months after being a long time SPG platinum. I was intrigued by having a single program due to my travels sometimes taking me to the hinterlands and SPG coverage outside of cities is poor. I did about 100 bed nights with Marriot over that time, plus 7 or 8 small meetings (10 elite nights is a great benefit). 80% of my stays are at Ritz/Luxury Collection type properties in a few locations and it covers me making platinum each year. This year I went back to SPG full time this year.
I learned a few things in my time in Marriot land.
SPGs customer support is light years ahead of Marriot's. Small issues with Marriot would be painful to resolve on the phone and major issues could be sorted out via Chat with SPG. Lately, I tell the service people at SPG I hope they take over or at least influence Marriot when things are combined, but I am not hopeful.
Marriot low-end properties (Courtyard/Springfield-Fairfield Inn/etc...) really suck. Granted SPGs are not that great either, so I'll just be a free agent in the hinterlands and book the best hotel be it Hilton or Best Western (Had decent stays with them, lol) or Marriwood.
Marriot has some great boutique properties.
Redemptions in big cities are better for Marriot than Starwood. Redemptions in peak vacation destinations are vastly superior with SPG.
Platinum recognization is better with SPG.
Points generation is better with SPG.