I am surprised the Dolphin came up higher than the Swan. The Swan used to regularly be about $20 a night more than the Dolphin before renovations were complete. This summer they were the same price. The Swan is a Westin and the Dolphin, a Sheraton. Westins are usually considered to be more luxurious though I don't think that's the case with these two. The Dolphin could possibly have a convention booked so discounts aren't showing up.
If it makes any difference, we wanted to book the Dolphin this summer because we loved our first stay there. It was full, so we stayed at the Swan, and liked it even more! The Swan is smaller and easier to get around. You do have to walk over to the Dolphin for Tubbi's and the Dolphin has a more impressive lobby. It actually took us less time to get to the Grotto pool from our room at the Swan than it did from our room at the Dolphin. When we were at the Dolphin, we were on the East Wing, so it was a longer walk to get anywhere. The Swan also has 2 sinks in the standard room and queen beds vs. oversized doubles at the Dolphin.
You will love either hotel. They are right across from one another, so you can easily enjoy the amenities at each one.