September and October tend to be slower months than December. I have been to both parties.
Here's how I (an average Joe who has attended both several times) compare them.
Both pretty much both offer the same extras, Character Meets, Special Shows, Special Parade, Special Fireworks, Free Snacks and special merchandise and don't forget your Sorcerer's of the Magic Kingdom Party Card which if you don't play you can easily sell on eBay for $10 or so after the party to offset your cost.
1) Character Meet & Greets - Character's you don't usually find otherwise can be found at a party, both MNSSHP and
MVMCP are equal on this
2) Shows - Both have special shows, I feel that MNSSHP wins out in this category with the Hocus Pocus Villain Spectacular
3) Parades - This is where I feel the MVMCP wins out with the better parade
4) Fireworks - in years past I have preferred the MNSSHP fireworks over the MVMCP, they are changing it this year so we will see
5) Food - not the free food (milk and cookies vs candy) but the specialty foods for purchase. This comes down to if you prefer Halloween vs Christmas as both are excellently themed
We usually don't bother with the trick or treating for MNSSHP, find out ahead of time the best cookie for MVMCP and usually don't ride rides (though we'll jump on some if there is a 5 minute wait or less) We are there for the extras you don't get during regular hours. For us we book our vacations to coincide with the parties if we are able and getting to dress up for the MNSSHP is treat in itself as adults don't get to usually do this. This year I am thinking of going with a "Line starts here for Galaxies Edge" costume