If you have a car, I'd strongly recommend using it as much as you can. Niseko is a cool spot and a great mountain, but unfortunately has become a victim of its own success. Too many people so the powder gets tracked quickly and the lift lines can be long. If you're up early and know where to go, of course you'll still get great powder because it snows so much, but it won't stay fresh all day unless it's dumping.
As mentioned, Moiwa is a smaller ski area just to the side of the main Niseko resorts and is worth a trip.
Rusutsu is definitely worth a trip and is less than an hour from Niseko. Its a big area that's less crowded and has a lot of options to find fresh snow. The good thing about Rusutsu is that even if you don't know the place, you can more or less go anywhere without worrying about getting lost or stuck. All the lines funnel out to a lift eventually.
Also worth checking out Kiroro. Slightly flat in places, but there are some good pitches and the place gets ridiculous snow.
If I were you and had 2 weeks and a car I would head to Niseko for 5-6 days initially to cover Niseko and nearby resorts. Then I'd head up north to Asahikawa. Stay there for 3 or 4 days and hit Asahidake when the weather's good, and also Kamui Ski Links and maybe Furano if you can get a local to show you round.
Then finish off your trip in Sapporo and hit Sapporo Teine and Kokusai, two of the local city ski areas. Teine has the steepest inbounds powder on the island and Kokusai has some of the lightest snow.
Asahikawa and Sapporo are both large cities, so its easy to get cheap accomodation in business style hotels then travel to the ski area of your choice each morning.
If you have any specific questions, feel free to ask here, or message me. When you're in Sapporo, if I'm free, I'll happily show you some good lines at Teine.
