Hmm tough call...if you go during a promotion, the discounts at BC tend to be better, if that matters.
Personally, I think the Poly is the BEST location of any WDW resort. You can jump right on the MK monorail from the lobby OR you can easily walk to the TTC and go directly to EPCOT. During our stay there last month, we were in a building very close to TTC and it was AWESOME. Also, not sure when you are going, but the Poly is currently be updated, so you could have the benefits of freshly refurbished rooms. Other things to consider, I find that the Poly tends to get a bit crazy in terms of crowds. The Lobby is frequently packed b/c of 'ohanas, especially in the evening. The pool is smaller than BC so it gets crowded during peak times. I LOVE walking around the Poly at night or sitting on the beach to watch wishes. If the MK makes your disney vacation magical, then the Poly is an excellent choice. Some people claim the Poly is dark and outdated but, for me, it's iconic Disney World.
The BC is one of our FAVORITE resorts. The pool is fantastic. My kids love it and we can spend the entire day at the pool and not get bored. I find the resort to be very clean and bright and inviting. The vibe is fun with a party atmosphere....especially at the pool. We love being able to walk to EPCOT and HS. Also, I think that area is great for older kids or couples, because the Boardwalk is lively at night. It's a great area to jog if that's your thing.
Aside from cost, we tend to plan on trip on the season...during a "cooler" weather trip where we are not guaranteed pool weather, we are all about location to the park and convenience. So during our February trip, the Poly was a no brainer and worked out great for easy access to the parks. During a warmer trip, when we are guaranteed pool weather, I think the BC pool is better than the Poly, so we would chose that any day. Either way, both are awesome resorts.