To be honest the Dolphin has all the emenities (laundry, closer to the grotto pool, Tubbies, car rental, etc, Camp Dolphin - daycare, etc...). It's a much bigger hotel too. When we stayed at the Swan thinking back it may have been a bit better at the dolphin for getting to the things we needed. The only downside I'd give the Dolphin over the Swan was that it was much busier/louder. However you may not even notice. These 2 hotels are so simular that it's really no big deal one versus the other.
Have a great trip.
Andy