Poly. WL is a great resort (and it's just hard to beat it for Christmas Holiday theming), but the intangibles for disability are so much stronger for the Poly. Plus the fireworks are much more viewable from the Poly. But for using wheelchairs, it effectively gives you pretty easy access to the restaurants at both the Grand Floridian and the Contemporary. You should also be able to access the Ticket and Transportation Center fairly easily by wheelchair and ECV; the resort monorail from the Poly goes GF, Magic Kingdom, Contemporary, TTC and then Poly, meaning that almost every time it will be faster to just roll over to the TTC if Epcot is your plan for that day. One advantage the WL has over the Poly (as well as the GF), is that if you're staying in the hotel, or the Cooper Creek villas, not the other DVC villas, you can get to and from registration and the common areas without having to go outside.
Now, as to old rooms versus new rooms, don't take that into consideration right now. WL is in the midst of a room refurbishment (not sure if it's finished), so room freshness is not going to be an issue. But room size is: The Poly has bigger rooms, and that makes a difference for wheelchairs. The Poly seems like the best choice for you and your family. As for us, even when not staying there, the Poly is always part of our trip, dinner there (usually 'Ohana), character breakfast at 'Ohana, and grabbing Pineapple Dole Whip outside the MK. For us, the Poly wins in every way, except for the holidays, where the theming, massive fireplaces and atmosphere at the WL easily win.