Have stayed at both. Swan's rooms seem a little smaller when you have 2 beds because of the queen vs. double beds in the Doplhin. Swan seems a little more formal in design, vs Dolphin I feel is cheerier and more Disney like. Also, as someone stated prior, the laundry and Picabu and the Dolphin fountain are all in the Dolphin. I found myself having to trek over to the Dolphin while staying at the swan, and not vice versa when I stayed at the Dolphin.
Oh, in coming back on the bus from the parks, you stop at the Swan first before the Dolphin which was nice having cranky kids.
Both are great hotels, so you really can't go wrong with either. I would probably choose the Dolphin next time, but it is also alot bigger with longer halls and more crowds. I like that there are more resources there than the Dolphin.