Never stayed at Poly, but I do want to point out a few great things about BC (I stayed there in '08)-
1) the pool, Stormalong Bay, is the best on property without a doubt
2) BC is a lot smaller than Poly, so less walking
3) Poly may only be a monorail ride away from MK, but BC is a quick walk to Epcot. can't beat that
4) You can see Illuminations from some parts of BC and also the bridge across from the back entrance
5) back entrance brings me to my next point- it is a much quieter and quicker entrance than the one in the front. yes, you are dropped off between France and UK which may be inconvenient if you plan on spending a lot of time in Future World, but it's a way to get in the park hassle-free
so there you go. nothing against the Poly, but i would probably choose to return to BC if it were up to me