We just got back from the BC, and we loved it. The Poly has always been my favorite resort, and there's nothing like it, IMHO--the theming is the best. But in all honesty, there were things about the BC that would make me choose it again over the Polynesian. SAB is an awsome pool, it is huge and there's something there for everyone. We loved watching the movie on the beach one night, and the campfire sing-along and marshamllow roast, too. The thing that surprised me the most was the convenience of everything--the pool, the lake, the Boardwalk, not to mention EPCOT and MGM. It's just not spread out at all. The hotel itself is more compact--all the rooms are in one building, not in separate buildings like the other resorts we've stayed at. it may be considered a large resort, but it just seems smaller and more cozy. We loved being able to walk into the lobby and go right up to our room.
However, my children are older now--15, 13, and 8. This was our first trip without a stroller and diaper bag, so that makes a lot of difference. If you're spending a lot of time at MK, the monorail would be convenient, but it wasn't a long walk to the bus stop or marina to hop a boat. When we stayed at the Poly, we had a great room that overlooked the marina and pool, but it was a bit of a walk to catch the monorail. At the BC, it was a short walk to the bus stop from the hotel lobby.
As for missing the view of the castle, I loved seeing the Boardwalk at night, and it's closer than the castle.
Either place will be wonderful.