The buffeteria, laundry and grotto pool is at the Dolphin. You can only get your mug refilled at Tubbis at the Dolphin.
So, it will depend on who is going and what you want. We stayed at the Dolphin in June with me, DH and my 2 girls (11 & 8) and we much preferred the Dolphin due to its location of the above mentioned things. But if you are going with no kids, or don't plan on using those things, I might stay at the Swan with the newly renovated rooms and the Heavenly Beds.
Check out the Swan/Dolphin FAQs that is a "sticky" at the top of the Resort board. It gives lots of information that might help you make your decision.
Maggie