I’ve stayed at both the Marriot and Sheraton. To me, it’s the Sheraton hands down. The grounds are much nicer and you’re on the beach. Yea, it’s a walk across the sand but you’re on the sand.
The Marriot pool is decent and the outdoor bar is nice but I give the Sheraton the edge all around. That being said, I am PLAT and get to use their awesome club lounge and am often upgraded to a suits.
The Sand Key Sheraton is one of my Hidden go-to places for a quick getaway. While the Marriot is certainly not a bad, IMO, the Sheraton is better.