We've stayed at both for 10 days each, the BC last August and the Dolphin the August before that. I'd recommend either to be honest, I guess it all depends on what preferences are and whichever is the best price, if that's a consideration.
Both are equally nice for location, the Dolphin being in better proximity to MGM and BC being in better proximity to Epcot. Both are ideal for all, however, since we walked to Epcot easily while staying at the Dolphin and also walked to MGM easily while staying at the BC. We like to walk, but obviously each has the boat transportation as well and it really is fantastic.
As for amenities, both hotels offer wonderful pools and all the services one would expect in a deluxe category hotel. The service is likely hit or miss as I've read both positive and negative reviews of just about everything at either of these hotels. While at the Dolphin, the room itself was a bit frayed and worn, but the housekeeper was wonderful and things were kept very clean. Every night she always arranged my daughter's animals in funny poses and really went all out. At the BC, however, the room itself didn't really look worn, but it wasn't fully cleaned when we checked in and we really only got what needed to be done at minimum throughout our stay.
That all said, I love both and would recommend either since you really can't go wrong with either one. My only advice is that if you can get a better deal at one over the other, and the money really is a consideration, then go with what works and don't sweat it!
Good luck!