My most frequently asked question...
While it is true that Mammoth is so high it can get snow in October when Tahoe is getting mostly rain, the entire Sierra is prone to month+ long droughts, which are quite unpleasant when they occur in November/December, most recently in 2006. The Sierra has been on a nice run of seasons recently, but there were a bunch of slow starts back in the late 80's/early 90's.
The problem with Banff/Lake Louise is that natural snowfall is quite modest though preservation is excellent with the high latitude and cold weather. Thus a good choice in late season, but not so reliable early on, especially for steep terrain like Whitehorn and Delirium Dive.
If you had to pick, Grand Targhee or Wolf Creek.
The polar opposites in illustrating the impact of weather volatility. Targhee is the most consistent snowfall location in North America. Therefore it's the only place with a perfect Christmas reliability record, and realistically that reliability is well in your favor back into early December. For October/November you need to be lucky, so you want a place with volatility that gets a lot of snow at once in big storms. Wolf Creek is a perfect example of that, and has an even higher base elevation than Mammoth, so if the storm comes any later than ~Oct. 15 the odds strongly favor it being snow not rain. The flip side of Wolf Creek's volatility is that it's still bone dry at Christmas ~10% of seasons.
As always the best advice is:
+1 on being spontaneous with your choice before January in the west.