I have stayed at all three resorts, and if I had to choose one, I'd go with the Poly. WL was our first deluxe and while my kids and I truly enjoyed it, our stay at the Poly this last December was one of my all time favorite trips. The location of the Poly was second to none. We loved the atmosphere and the ability to watch fireworks from the beach. We were steps from the ticket center monorail, and the convenience to MK was awesome. If you want a vacation from a vacation I would think about WL. It's kind of a pain to get to MK (some will disagree, I'm just not a big boat person), but everything else was enjoyable. The lodge feel is nice, but the rooms are a bit small. I've stayed at both POR Riverside and French Quarter. Love them both, but prefer French Quarter for it's size, Riverside for the food court and atmosphere. In my opinion you can't really go wrong...
Are you going with your family, couples trip, solo? Those kind of things would make a difference. Also, if your looking to save and wondering if you'll still have a great experience I would book POR no question. Good luck!