I actually couldn't decide between BWI and the Poly ... so we're going to do a few days at both.
I love the Poly, but BWI offers great ease of getting to Epcot, particularly WS for dinner, boats to MGM, evening entertainment, and fun restuarants. I don't know what type of room you'd have, but we're staying in a deluxe concierge room at BWI because I liked the idea of extra space and a guarranteed boardwalk view. That's also a consideration.
At the end of the day, though, lagoon view at Poly is pretty amazing.
I think either way, you'll have an amazing time.