are you looking at least year's sizes - this year Burton is showing 142, 146, and 150 for Nug sizes.
I would say that if you're getting this board as your only board i'd get the bigger size but if it's a 2nd board go small, it's kind of the point of it.
I rode one last year in spring conditions; it was a lot of fun even if you're not a park person, really easy to turn and easy to ollie and spin and so on. Personally when I think powder I think of a tapered board with a lot of setback or maybe something powder oriented but twin like the Sherlock, which is very floaty. I have a sherlock which I bought for trips out west, I doubt that if I had a Nug and I was told there was going to be deep snow I'd take it instead but I haven't ridden the Nug in deep snow so who knows, lots of fresh at whistler latlely, post a review.