Hey,
Went to Niseko and Happo One / Cortina for a month back in Janurary and am going back to Niseko in december for the whole season (dec - april).
There are 2 areas to chose from (actually more but for simplicity ill condense it to two)
Niseko:
North Island
Slightly better snow quality (gets the majority of its air from the sea of Japan)
more constant falls
fairly mellow terrain
open gate policy
some option to hike some decent hills
best resort access backcountry in Japan
better maintained terrain parks
Hakuba:
Central Japan
Slightly (ever so slightly) heavier snow (gets air mixture from sea of Japan and pacific ocean i believe)
more clear days between dumps (not as consistent)
Far bigger and steeper terrain, hakuba is located in the Japanese alps and you can ski some super nice steep stuff there, unlike niseko
Due to the steepness and high snow levels there is a real avalanche danger so the resorts are extremely tight about skiing out of bounds
less quality terrain parks