The Dolphin has a very impressive lobby, Tubbi's cafeteria,Dolphin Fountain Ice Cream Shop, closer to the grotto pool (usually), laundry, and an arcade. The Swan is a Westin (vs. Sheraton) and is completely renovated with the Heavenly Beds. The Dolphin is almost but not completely renovated. The Swan is much smaller and some people like that. Even though the Dolphin has more amenities, the Swan is so close it doesn't matter much. I imagine it's closer going from the Dolphin to the Swan than all the way across the POR resorts.
I do like the Dolphin a little better, but we are staying at the Swan this year because the Dolphin was booked. Some minor differences are the Swan has 2 queen size beds and the Dolphin has 2 oversized double beds. The Swan also has two sinks, one in and one outside the bathroom. The dolphin only has one sink outside the bathroom. The Dolphin room is a little bit bigger square footage wise, but with the layout it's not really noticeable. The suite options are slightly different between the two.
You can't go wrong with either one.