I live in Japan and ski every holiday, weekend, sick day in the season and I have travelled across Japan to ski its different resorts.
You are right, Hokkaido (the north island) is the best bet for snow cover at christmas time.  This is the home to Niseko, Japan's most famous resort due to it being owned in part by a western company.  The snow is great, in abundance and reliable.  The terrain can be tame for an advanced skier but that early in the season who is doing big drops anyway?  There are other resorts in Hokkaido that can also be fun.  Asahidake (which is featured in a Tanner Hall vid, I can't remember wich) is basically a tram to the top of an un-groomed mountain.  It has flat parts and is best with some hiking gear (and a guide).  Around Hokkaido's largest city, Sapporo, there are some fun options.  Teine (an olympic host mountain during the Sapporo olympics) has some great steep bowls off the back, Kokusai will actually have park features even that early as will Kiroro.
On the main island (south) Hakuba is Japan's skiing crown jewel.  This is Japan's center for steep tree skiing, pillow lines, cliffs and big open faces.  The snow is a bit heavier than in Hokkaido but that actually creates a better base for its more textured features.  Hokkaido may be more northerly but Nagano resorts are higher so the snow season is roughly the same but, like I said above, in the early season, SOMETIMES hakuba can be a bit low.
Ddue to altitude and a better base, south island resorts often have a longer season.  The parks are much better on the main island but not actually at the Hakuba resorts.  For park, the resorts in Gunma and Niigata are much better.  Be warned, it is here were the most snow in Japan falls and it is a heavier snow at all but the highest resorts (eg. Kagura or Tenjindaira) so the parks can be burried for days.
Other options in Japan include the backcountry resort Hakkoda (like asahidake but on the main island) Zao (flat but pretty), Yuzawa (close to Tokyo), Shiga Kogen (biggest resort in Japan), Nozawa Onsen (very pretty old-school town and great BC terrain), Okutadami (park specialised mountain with lots of pros, only open in early season or spring due to too much snow) amongst many many others.