Some hard facts.
Facts:
Poly on the monorail, close to MK, Monorail to Epcot. Bus to Studios and AK.
BC next to Epcot. Boat or walk to Epcot and Studios, bus to MK and AK
Poly-Ohana and Kona Cafe, easy access to Cont and GF restaurants
BC-Cape May Cafe, Beaches and Cream, easy access to Yacht Club, Swan, Dolphin, Boardwalk restaurants.
Poly-Zero entry pool with volcano and waterslide.
BC-Mini water park pool with interactive areas and huge slide. Big reason for people to stay here.
Rooms at Poly are bigger.
Poly is much older but both resorts have had major room rehabs.
Poly is multiple buildings and has more of a "hawaiian village" feel. Rooms still open to a hallway, not outside.
BC is one building more of a large cottage feel.
Both resorts are on a "beach" that you can not swim in.
Both resorts have a marina.
My opinions:
If you are spending more time at MK, stay at Poly.
I love the Epcot Resort area. I enjoy the walk around the lake to the other resorts and love to have multiple places to eat at the various hotels. (Same can be said for the monorail resorts.)
I spend more time at Epcot and Studios so would prefer BC.
I hate the bus system at BC. It shares with All 5 Epcot resorts. Last time I stayed at Poly, some busses shared with GF, and the monorail is a big plus.
As for guest service...I find that some of the older resorts have CMs who have been there for ever and really hold up the Disney standard. That can be hit or miss at BC.
Hope I could help.