OKW's rooms are MUCH larger. The resort is very relaxed & really has great themeing. I would say OKW without question. SSR is nice but for your group I would definitely suggest going with more room at OKW. In comparison to SSR, OKW's location is about the same to everything but DTD, but OKW also has a boat to DTD which is lovely & only adds about 5 - 10 minutes on the boat.
Yes, most of the buildings don't have elevators, but when you make your reservation, tell them you have a service dog & need a bottom floor or a building with an elevator & you won't have to worry. Of all the times I've stayed there I've always had the bottom floor.