I agree with HeatherFeather and Quiltymom. I don't ever advise someone to skip Disneyland vs. WDW if its a once in a lifetime trip. Because to me, Disneyland is wonderful and while not as big as WDW has a lot to offer and doesn't need to take a backseat to WDW. However, due to your location, it sounds like Disneyland will be in your future so why not venture to WDW if you have the time, money and desire. As somebody else said, you never know if the opportunity will arise again. Here are some other pluses to WDW in December:
Mickey's Very Merry Christmas Party. Wonderful, hard ticket event at the Magic Kingdom. Really fun for kids of all ages. It runs in December up until the big rush (last year that was around 22nd). You can check out the
MVMCP thread on the Theme Parks board here on the dis.
Candlelight Processional at Epcot: A beautiful event, truly one of the best examples of the "reason for the season" I have ever seen. And it runs most nights in December. Also, Illuminations during December has a spectacular, wonderful ending with Walter Cronkite narrating and a Let There Be Peace on Earth theme. I hghly recommend the dinner package. The meal won't cost anymore and then you are guarnateed a shorter line and entry into the theater.
The resorts. The resorts at Disneyland are decorated at the holidays but nothing like what they do at WDW. Its almost like having a whole new set of attractions to explore and enjoy. If you stay at the Poly, you can take the monorail easily to view the other monorail resorts. And if possilbe venture over to the WL as well as the AKL and the Epcot ones. They are all very decked out and lovely.
Dining: WDW has so much to offer with dining. Its really fun to eat your way around the world, and I am sure your mom would enjoy it. I know my parents did! Disneyland dining has some great choices but dining at WDW is way above and beyond what Disneyland can offer.
Increased entertainment. Unlike Disneyland, WDW never scales down their entertainment. Not sure if your before Christmas dates at Disneyland would have Remember and the Electrical Parade running more than on weekends (if at all with the Electrical Parade). At WDW, regardless of season, Wishes, Soectromagicm Fantasmic and Illuminations all run nightly. Because its a tourist destination vs. the huge local base that Disneyland draws in, there are slower/cheaper seasons but not truly an off season at WDW with a reduction in what you are offered.
Definitely stay 7 nights. You need that for WDW. And the Poly looks fabuolous, I really hope to stay there someday. I think you should give WDW a try. You won't be sorry you did I am thinking and it will feel familiar in many ways yet something new and different to try as well. Good luck with you choice. The good news is that you can't make a wrong decision, since any Disney vacation is pretty exceptional!