That's a tough call. Both are awesome.
I have a mate who lives in Nagano so when I visit he takes me to his local mountains. Which is Myoko Suginohara. Lots of places to go off course and get pow through trees ect. Not a bad park setup. Doesn't have much nightlife. That I found anyway.
Hakuba has more of a selection of mountains. Good nightlife. Easy to get to. And lots of pow and parks too. I think most of the mountains are steeper around Hakuba.
If I didn't have my mate to guide me around Myoko ,I would just go to Hakuba.
Im sure you would find both have what you want. I find Hakuba more tourist friendly.