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.
If you're a halfway decent boarder, you will spent little time on the Niseko resort side, and will be spending most of your time on the backside. And if you do, you will want a board with serious float.
I was there for a week last winter in Feb - it had some epic powder - more than waist deep.
If you plan to do the resort thing, any board will do. But not any board will work on the backside - you will get stuck and it will be hell getting down the bottom part of the backside which is somewhat flat.