For Niseko I'd go with the Chetlers. Sure it doesn't snow every day there but when it decides to come down, it tends to puke. 2 of my friends were on Obsetheds when we were over there and both commented that they wouldn't have minded a wider ski. Assume it'll dump, you'll enjoy it more on a big ski when it does.
Plus IMO go a little shorter - the wide open stuff gets tracked there before you can blink, if you want all day fresh pow you have to get into the tight trees where you sometimes need to be able to flick your skis around.
The park is pretty average so don't worry about that aspect. They make an effort but the jumps tend to be a bit odd. If you're not perfectly dialed for speed you either go way, way deep or hit some of the most vicious knuckles you'll ever encounter.
If it rains and the snow is all crusty just hit the onsen or go lap the airbag at Hanazono for a bit.
Oh and take a clear or really, really light lens for your goggles - the night skiing is sick, especially when it's dumping. Dropping skier's left of Miharashi in the dark (no lights out there, relying on reflected light from the clouds) while it's puking snow is a must-do in my opinion.
