Dolphin has smaller, double-sized beds (a minus) and is a big bigger and more complicated to find one's way around (a minus), but is closer to the Grotto Pool (a plus). Those are all tiny differences, however (unless one is tall enough that a queen-sized bed is a must).
I would say "buyer beware" with respect to the Swan and Dolphin, though. Marriott doesn't do a great job of depicting the room categories on its website, and the relies on its disclaimer that "room configurations may vary" to justify assigning you pretty much anything, regardless of whether it bears any resemblance to the room you booked. We booked a very specific room category (a premium studio suite) based on the amenities shown and listed for that room online, only to find when we arrived that our suite had an entirely different layout and size than the website had indicated, with major pieces of furniture and cabinetry missing from the room, no clothes closet, and a combined single bathroom with only one sink instead of the divided bath with 2 sinks that was advertised. I politely complained to Marriott, and they wouldn't even do me the courtesy of responding, let alone doing anything to assist us, so as much as we enjoyed the Dolphin otherwise, they've lost my business going forward. YMMV.