I prefer the Swan because I had a bad experience at the Dolphin. They had a huge convention and did not clean my room for two nights - I complained to the manager on both ocassions and they finally refunded my fee for those nights, which I felt was good customer service. But they were clearly unable to handle the large volume of guests, and for the remainder of my stay I had to literally put my trash out in the hallway because the cleaning staff would never empty the cans. It was still stacked up out there on the day I checked out!
The Swan is a Westin property and the Dolphin is Sheraton, so with the Swan you get the higher level of service that comes with Westin. Queen beds versus doubles and separate shower/vanity areas. The Swan is a tad more expensive and more quiet. The Dolphin is definitely busier and more active. The plus for the Dolphin is the quick service Buffeteria, Picabu's, located on the bottom floor. Extremely easy to grab breakfast on the fly each morning before heading to the parks, or for a late night snack. Swan only has sit down restaurants and no quick service - just a small area in the lobby with coffee and some danish in the a.m. Although it is a very easy walk over to the Dolphin to use their quick service option.
There are always deals for both on mousesavers.com or allears.net, or if you are a member of the Starwood Preferred Guest Program. It's a terrific location for people like us who spend the majority of our time in Epcot - quick walk to both Epcot and Hollywood Studios. Still have some Disney touches but not the scale you would expect to find in a Disney-owned property.