We've only stayed at the Swan - my reasoning was the sinks, larger beds, and the busses - With the extra sinks, one person can shower while another gets ready (Drying hair, make-up, coffee, etc.) at the outer sink area. This is great if you have a large co-ed group! I Think they are both VERY nice hotels, and I understand the difference in the beds is not that significant, since the Dolphin's beds are 'oversized' doubles, or so I've heard.
If you stay at either hotel, it is easy enough to walk through the other -so if you want to see the lobby, or eat at a particular restaurant, it is easy enough. But like the above poster said, if you plan on eating at Tubby's, you might be closer to it at the Dolphin. The food was fine there, but the name 'Tubby's' is a bit of a turn off personally...who thought up that one?
You can also dine at the Boardwalk or BC/YC eateries easily enough (The Boardwalk is a short walk, the BC/YC is a short boat ride away). A popular choice in the AM is the Boardwalk Bakery, though it can get a bit crowded at park opening.
For us, we didn't end up going through the front lobby of the Swan all that much, because the walkway/boats to Epcot (the boats also take you to MGM) and to the pool areas are through the back side of the resort. You do go through the lobby to get to the bus area. This is also true of the Dolphin, though you'd go through the side exit to get to the pools. (The grotto pool is a bit closer to the Dolphin.)
Another important factor for us was the bus service! The bus comes to the Swan first - so you are more likely to get a seat in the morning, and you get dropped off first at night (Though you do go to the Dolphin before heading to the parks). If the bus gets full, you might have to wait for the next bus at the Dolphin.
Last, might be the view: I'm not sure about the various views from the Dolphin, but if you look at the location, The Swan is closer to the parks. The deluxe view from the Swan can be looking towards Epcot...the standard view was looking towards the parking lot, but also towards MGM (TOT), though you might also end up with a view looking towards the Dolphin/pool area. The Dolphin is behind the Sawn, so while there are rooms that also look towards Epcot, it is a more distant view. Other views would be of the Swan, pool area, or the farside of the Dolphin.
Again though, either hotel is VERY nice, and overall I understand that they are very similar. The differences are not all that big. I'd happily stay at either!