I'm outnumbered here, but I vote BC...we went with our 2DD 8 and 4 recently and the 4 yr old loved the fireworks at EPCOT so much...it was nice to walk over for quick visits to EPCOT which is all we really did there...no full days, just dinners, princess breakfast ,fireworks on nights we were not at MK or MGM...
the pool at BC was fantastic...sandy bottom, lazy river....quiet pools at night...
when we went to MK it was for the day so back and forth on the monorail was not an issue for us....boat ride from BC to MGM was fun and different and EASY
I don't think there is a bad thing to say about either, but they could look at what they think they are going to do the most and how each would fit them better.....
Poly is a little older would be my only negative comment