We were also torn by this choice.
We have an 8 night deluxe dining vacation coming up where we have the following ADRs:
Dinner:
'Ohana
Citricos
Artist Point
Cali Grill
Hollywood Brown Derby (Fantasmic package)
Jiko
Flying Fish
Yachtsman
Breakfast:
Kona
Cinderella's Royal Table
Crystal Palace
Tusker House
Cape May
Lunch:
Les Chefs
Via Napoli
50's Prime Time
We're also planning on attending
MVMCP, Fantasmic (with dinning pacakge), and Illuminations.
Based on all of this, we decided to do a split stay between Poly and BC. We plan to visit MK (twice), plus MVMCP, during our Poly stay, and then go to EP on our Poly check-out day. Later that day, we'll check into BC, and during that stay, visit HS, AK, and do a second day at EP.
We'll eat at the MK area restaurants during our Poly leg and EP/HS and AK area restaurants during our BC leg.
We sort of view this as getting more of the best of both worlds (and less of the worst). For example, the two most often cited negatives of BC are: (1) limited CS options; and (2) poor bus service. But beause we're doing deluxe dining, we don't care about (1) and because we only need to ride the bus on our AK day (which we'd have to do regardless of where we were staying), we don't see (2) as a big negative.
At the same time, we'll still get to take advantage of some of BC's often cited advantages: (1) being able to walk to EP and HS (particularly nice, for example, after Illuminations); and (2) SAB (including hot tub, which Poly does not have).
That being said, if I had to pick just one of these two resorts, I'd probably pick the Poly. Overall, I like the theming and location better (we always do at least 2 MK days, so being able to Monorail to both MK and EP beats being able to walk to EP and HS, especially with the spotty bus service from BC to MK).