You're going to want something with float. I rode a 161cm Joystick there in January and I was wishing for a bit more at times but 90% of the time it was fun. Something like a camel toe would be spot on for Niseko. Plenty of guys there ride swallowtails, fish etc. You can have heaps of fun on a stiffer all mountain board as well, particularly if you were staying in bounds more.
My favourite spots were the hike to the summit, Annupuri bowl and gate 8 at Annupuri.