I love both for different reasons.
I love being able to watch Wishes and Electric Water Parade from the Poly beach, being able to see MK- The Castle, Space Mtn, GF & CR walking along the beach. Beautiful landscapping, has the largest rooms, and 2 of my fav restaurants.
Also has great transportation to MK, the resorts, and a walking path to TTC for Epcot Monorail.
The one thing I didn't like about Poly was the hallways, they need some TLC in my opinion, but wasn't a deal breaker.
Also love BC for the beautiful lobby, and bright pretty rooms, walking distance to Epcot and a 20 min walk to DHS or boat. Nice dining options- we enjoy Cape May and Captains Grille. Lots of dining options in Epcot WS too! You can walk over to The Boardwalk too. Hands down best pool on property. Also everything is under one roof at BC, so if you want to go get a mug refill, it's only an elevator ride away.
They are both great resorts, no bad choices here. I couldn't pick a fav.