The California Grill will still be open in December, but you may have a tough time getting a reservation. However, if your group is small, you can find space at either the bar or the counter of the open kitchen to sit, as those areas do not take reservations. The sushi at the California Grill is quite good.
There is "take away" sushi in the Polynesian Great Ceremonial Hut (lobby) up on the second floor right by the monorail entrance. That is all they sell, so it wouldn't be a place to go for people who don't want sushi, but others could do take out at Capt'n Cook's. (Capt'n Cooks also sells sushi, but AVOID).
Kona, (the sit down restaurant just to the right of the sushi counter in the Poly) also has sushi on the menu and it is good. And that restaurant serves plenty of other things as well.
Kimono's in the Swan Hotel has excellent sushi, but the rest of the menu is limited. Still, there is stuff there for a non-sushi eater.
If going just for sushi, my preference would be:
Kimono's (bigger selection than California Grill)
California Grill (excellent sushi and a great view)
Polynesian walk-away stand.
If going for sushi and needing a place with alternatives, my preference would be:
California Grill
Kona
Kimono's
All that said, I have not been to Wolfgang Puck's in a while, so I won't comment on the sushi, one way or the other. I am certain that others have opinions.