My answer changes depending on if the fireworks are more important or the snow.
For fireworks, the best spot is the hub, right in front of the castle with the second best spot being along Main Street, closer to the hub. As long as you can see the whole castle, you should have a good view of the fireworks. Both of those places will also have snow with Main Street seeming to be a little better than the hub.
For snow, the best place we have found is along the Small World mall area (where the princesses are now). The snow machines seem to be hit or miss around the hub. You can see it coming down, but it doesn't seem to cover the whole area. But near Small World, there was tons of snow and people were getting covered in it. It made for some cute pictures where it really did look like it was snowing. The fireworks are not as good from this location though.
Note though that these snow reports are from the last few years so they may have moved machines around, etc, for this year.