It's been a little while since I was at MNSSHP but since I finally experienced MVMCP, I can finally compare!
I pick Halloween. And I'm not a Halloween person! While I don't care for the villains or the candy, the vibes of the party are much more enjoyable. I like the food better. I like the excitement better. The ride choices for the holiday theming is also just better. Characters are somewhat the same. They wear different costumes; Seven Dwarfs are in their normal clothes, Mickey and Friends wear Halloween costumes, etc. But you can see characters who are "autumn/Halloween" themed other than the villains.
No idea how Disney classifies that, by the way.
The food is often really great. You can see a lot of characters you haven't seen out in a long time. The magic shots for photopass are often something I actually want. Boo to You is one of my favorite parades. I think MVMCP has nicer merch (though this year looks like MNSSHP did a great job), but... that's really it. I'm just going to repeat the things everyone else says, really.