If the kids are along, we prefer Dolphin since it has the only laundry, arcade & quick service restaraunts out of both properties. It is also closer to The Grotto Pool. If it's just adults, then whichever is cheaper (usually Dolphin anyway).
Swan has 2 queens and 2 sinks in the room, that is the main differernce over rooms at The Dolphin. Both rooms are the same size. Overall Swan is a smaller resort, and has a more intimate feel. Dolphin is larger & has more of a hustle & bustle atmosphere.
You can't go wrong with either. They are connected by a very short walkway, and the boats to BW, BC, YC, Epcot & DHS are located directly between them. The walk to BW, Epcot & DHS is a few hundred yards shorter when staying at Swan. The walk to YC & BC is a few hundred yards shorter when staying at Dolphin.