One more consideration for your planning: The week before and after Chinese New Year (in 2013 CNY is the week of February 10) can get quite busy because Niseko gets a lot of visitors from China (including Hong Kong) and Singapore.
Thats interesting about Chinese NY. We usually go to Niseko first week Feb and were planning to do the same this year. I do not fancy sharing the accommodation and resorts with more people than usual though so may go last week of Jan instead. When did Chinese NY fall this year? Does it vary much? If it was around early Feb this year then we didn't notice larger numbers.