The only reason I would select Sapporo in winter would be to see the famous Sapporo Snow Festival...but next year's will be held in February.

Since you have the opportunity to fly anywhere for the same price, I'd really consider taking the opportunity to go off the beaten track and fly to Naha. Pleasantly cool (not cold) weather, World Heritage sites, plenty of US/Japan fusion culture, and lots of things that you can't see anywhere else in Japan. And you can just rent a car and be free of the confusion of trains (traffic can be a bear in the main city in the summer, but maybe not quite so bad in the off-peak winter time)

And for future reference, you can get round trip from Narita to Tokyo for $65 by taking the local line, though it would be an hour and a half trip.