Oh, this is always a tough one for me. I've done both.
I've done
MVMCP 3 times now last one being this past December. I did
MNSSHP the year before for the first time and we'll be attending it again this October!
I like them both.
I like MVMCP for the Christmas atmosphere, the snow, the decorations, the parade, the special shows and the parade. It's so magical and fun.
I liked MNSSHP for the decorations (and music piped in), the trick or treating, seeing the costumes, and the special parade. This is actually the only time we saw any Halloween decorations so that was nice.
Now if I really had to choose, I think it would be MVMCP. You just can't beat the Xmas season at WDW. I totally enjoyed MNSSHP too but it's just not the same. My DH enjoys MVMCP because he says Disney shouldn't be Halloweeny or scary at all cause it's Disney after all. Well, MNSSHP wasn't scary so I'm not sure what him meant...
Anyway, if someone twisted my arm it would by MVMCP. But I'd like to alternate each year if I could.