I agree that if you were really looking forward to the BC and the extra cost isn't an issue, I'd keep it. We have stayed at both the Poly and BC and while we did enjoy our BC stay, the Poly is our favorite. With that said, the Poly would be my choice (just my preference for atmosphere, room size, pool, etc), but it sounds like you are very excited about staying at the BC so I'd go with what you really want. The MK area is our favorite, but if you will be spending a lot of time at Epcot and DHS, you can't beat the location and close proximity to both parks. I can't comment on BWI as we've never stayed there.