We stayed at the Poly in December 2009 and 2010, and this year we stayed at GF. They are both terrific places to stay. Both resorts are beautiful.
GF is definitely beautiful at Christmas. I love the piano player and the big band in the lobby in the evenings (but be aware that the lobby can be absolutely packed and crazy in the evenings) I also like that the grounds are very well lit up and open and that I feel perfectly comfortable walking around by myself after dark there (I don't feel comfortable after dark by myself on the Poly grounds) The staff is great at GF. The rooms we had at the Poly were a little bigger, but not really that noticable- definitely not a big enough difference that it would be a deal breaker. The beds were much more comfortable at the Poly. Maybe we just got a room with some bad mattresses, but the GF beds in our room were hard as a rock and one sunk in at the middle, so that DH and I both rolled into the middle to the point that we wound up switching beds with DD after a couple of days. Her bed was better, but still not the greatest as it was still pretty hard. The pillows were hard too, but after the fact I heard that you can contact housekeeping and request down pillows, so that should solve that problem. This trip is the first time I can recall really having trouble sleeping at a WDW resort. Of course, after a few days at WDW, you are so worn out at night that you can pretty much sleep anywhere anyway.
The main difference, and one that definitely leaves DH and I prefering the Poly, is the transportation. (DD (5) would still tell you that she adores the GF over anyplace else). We have stayed at GF, Poly, WL, AKL, and POR and the bus service at the GF was the worst that we have ever encountered at any WDW resort. Frankly, I don't find that acceptable for the prices they charge for that resort. We never waited less than 30 minutes for a bus, and it was not unusual to wait longer. We were definitely never the 1st people there waiting even though it took us over half an hour. GF shares buses and it is the 3rd bus stop (Contemporary and Poly are the first 2). It was not unusual for the buses to come through completely full, and the driver would just open the door and wave and tell everyone they would have to wait for the next bus.
The boat service to the MK was great. Only one time was there ever any significant wait. We went to
MVMCP 3 nights (I know crazy- it was an impluse decision during the trip). 2 of the nights they were running 2 boats; however, the 3rd night they were running only the one boat back and forth and the boat line was crazy (as was the monorail). The people 2 parties behind us had to wait for the 3rd time the boat docked back to get on (they got cut off, the time we got on the boat) and the line was pretty huge behind them.
At the Poly, you can walk to the TTC and get the express monorail to Epcot. At the GF you have to take the resort monorail line to the TTC and then the express monorail to Epcot. When we stayed at the Poly, it took about 20 minutes to get to Epcot. From GF, it was closer to an hour. We did enjoy the monorail ride though, but it sure was nice to get to and especially back from Epcot quickly from the Poly.
Finally, one of our mornings at GF, the resort monorail line was down. It was a disaster at the bus stop. A big mob of not very happy people with long waits trying to get to MK or Epcot. If it had been the Poly, everyone could have just walked over to the TTC as I'm guessing the other monorail line was operating.