I was at Niseko last year in December for a few days. Stayed at a hostel close to Hirafu. All the resorts are connected at the top and you can get an "all-mountain" pass. The hostel I stayed at did offer a shuttle service but we had a car, which I highly recommend so you can go to an onsen (hot springs) after a long day. Probably the second best part of the day after the pow.