We love Epcot, so definitely BC. It's much closer to Epcot. I know YC is right against BC, but in reality YC's rooms are very spread out, so some rooms are a much longer walk and can be twice as far as BC to Epcot, if you compare the closest room at BC with the farthest room at YC. The walking adds up if you like to go back and forth several times a day -- it's the whole benefit of staying at a Epcot hotel. I like to do that for no apparent reason, just to feel like wow I'm living right next to Epcot. I also prefer BC's food court, which has an outside sitting option.
However, there's no denying that YC has advantages over BC. The rooms are much more beautiful. The balconies are nice and big. It's closer to DHS. So, staying there isn't the end of the world.
But BC is closer, and that's worth everything.