Only been stayed at Poly once, love the theming and the restaurants. BWV, several stays there. Here is how I would personally address the place to stay
Poly...proximity to MK, 2 stop ride to Epcot, walk to TTC, close proximity to other resorts(more restaurants to choose from)
BWV...2 parks in walking distance, a few good restaurants in walking distance
Overall, BWV would be the draw for me, nothing against Poly, just a few more amenities at BWV and surrounding area