They are both fantastic! I think it comes down to which parks you will visit most. BC just can't be beat for EP and DHS. I took my DS3 to BC this summer and had no issues with SAB. We simply stayed in the kiddie pool area which he loved and there was no chance of him wandering in to he deep end. If you will be in MK a lot then Poly is the way to go! Also, since it sounds like you've never stayed at poly, you should give it a try! The rooms were refurbished about 6 years ago and the grounds are impeccably landscaped - gorgeous!!