Here would be my reasons for saying at each, but upfront, I have never stayed in BLT or Poly studios, so can't speak to the sizing. The proximity to parks would be my draw
BWV simply for convenience to 2 parks
Poly for convenience to monorail(you can walk to TTC which allows a little quicker access to Epcot)merits a quick ride to MK
BLT for 10 min walk to MK
Me personally, and I own BWV pts, I would choose BLT. Here is why...in addition to the mentioned above, I also like the closeness via monorail and boat to POLY, GF, WL and FW, which each have some unique restaurants. Plus, Contemporary, just a walk away, has the same, and wonderful other amenities to boot.
If not, BWV is my next choice, for reasons mentioned, plus a few good restaurants in walking distance around BW area, BC, YC and Swan and Dolphin. And I take advantage of being close to Epcot for slipping in for meals.
Poly, 3rd choice, don't get me wrong, I love the feel of the place, and Kona and Ohana help to make it better, but its still not at top of list