The Swan has queen beds while the Dolphin has doubles. However, you easily fit a rolloway in a Dolphin room since the beds don't take up as much space. Dolphin has a much larger, busier lobby. The Dolphin just seems much more active overall while not being more crowded. The Swan is quieter.
I prefer the Dolphin. I like the busier atmosphere. I prefer two doubles and a rolloway to two queens. Also, we seem to have had much better luck getting upgrades at the Dolphin.
I like them both though and would happily stay at either.