Chihster,

I've been to niseko in the winter a few times.

In terms of whether to rent a car, my advice would be against it. I have a lot of snow driving experience and many hours on steep snowy roads in mountains and I wouldn't take a chance driving in the dark on unfamiliar roads in possibly deep snow. Most of the ride is rather flat, even when you get to niseko, but the snow can come down hard so I wouldn't chance it. The bus service is relaxing and good. I take the whiteliner though not sure if they stop at your stop.

In regards to the walk with a snow covered road a three minute walk could be miserable with luggage. If you have wheeled luggage it won't work. You can ask your hotel if they can pick you up or ask what other arrangements they can make. Whatever cost will be cheaper than renting a car and you won't need a car after you arrive if your hotel is near a niseko united bus stop.

If you arrive on an international flight is give yourself time to catch the bus. Most buses leave from the domestic terminal so customs baggage and the walk in between will be cutting it close. CTS is on par with SIN in quality in my opinion, just smaller, so extra time can be spent well.