I can't speak for Whistler, but I was at Fernie Dec 10th or so last year.
It was still in early season conditions, meaning the main runs were open but some of the bowls were closed. We had a slow start last year though so maybe December is typically full bore?!? Also Fernie can get rain too, though not likely in December.
For night life, it's pretty tame. A couple bars/pubs in town but otherwise best to hang out in the hot tub and prepare for another day on the slopes.
You can get really good on mountain lodging deals in December. We've stayed at the Wolfs Den a few times now, and it's certainly not fancy, but it's got everything we needed (including a shared BBQ on the back deck) and I think the rate last year in Dec was something like $50-60 a night (in mid season it's typically in the high $100s).
Westjet has a seat sale on right now too. If you're coming from Toronto check prices out of Hamilton airport they can be upwards of $100 cheaper depending on the flight. Airport fees are lower there. I'd be tempted to book the flight but hold off on booking any hotels until a little later. If Fernie hasn't been dumped on you could stay in Banff instead and board at Lake Louise and Sunshine. Or do a few days there and a few at Fernie.
BTW all of the above mentioned hills have excellent learning areas. Pretty much the whole lower half of Fernie is a learning area, tons of terrain that's not too steep, mild Blues and some greens. Lake Louise is great but the main lower lift area can get busy. Nakiska would be another one with tons of learning space.