We've stayed at both, enjoyed both, but they really are quite different.
Poly -- Much larger rooms, amazing view (if you pay for MK view), great location next to pools, large lounge with nice views, boat and monorail to MK, walk to TTC to take the monorail to Epcot. Nice beach area, good restaurants... Dark color decor, multiple buildings (so you get wet if it rains).
BC -- Rooms are smaller but still very nice, lounge is on the 5th floor so not so convenient from the pool area, smaller lounge, walk to Epcot and DHS (or take the boat). Back-door entrance to Epcot. Best pool on property (SAB is much larger and more elaborate than the volcano pool), access to lots of great restaurants... Light and bright decor, and all in one building.
It's hard for me to really say one is better than the other -- they both have unique things they offer, and the BC CL is considerably less $$$ than the Poly is (at least that's the case every time I booked). The theming and look of the resorts are very different from one another -- that may help you make your decision.
As for CL, I thought the Poly had better dinner apps, while the BC had a slightly better breakfast and snack time treats. We liked the CMs at both lounges.
Good luck with your decision!
