I can't speak for Sheraton Safari, but I have stayed at the Dolphin - its nots my favorite by any means, but it is convenient as heck. The hotel itself is fine - like staying in a fairly nice hotel - we didn't eat anywhere there at the Dolphin, but we did appreciate the convenience of being there at Epcot.
With Epcot being open the latest of all the parks - it makes me appreciate the location of the Epcot resorts!