I'm in the minority, I know, but I'd choose
MVMCP over MNSSHP.
We had a less-than-stellar experience at MNSSHP in 2014. Part of that was due to my son being intimidated by the Halloween theme - none of us expected that until we were there and he was nervous and uncomfortable and scared. Obviously not the fault of Disney, but his nerves definitely had an impact on our experience. Part was due to the crowd - it was a sold-out night, so everything was pretty packed, and we ran into some really unpleasant, entitled individuals (like a "lady" who showed up moments before the parade, pushed a pair of 10/11 year olds out of her way to stand right at the rope, and whacked my 5 year old son in his head with the camera she was wearing on a strap around her neck, then scolded him for getting in her way). All in all, we ended the evening with a sour taste in our mouths about the whole experience. (Though Hallowishes was lovely.)
By contrast, we'd done 2 MVMCPs, in 2011 and 2012, and really enjoyed both. I don't think either of the Christmas parties we attended were sold out, and that probably made some of the difference, but we felt that the vibe of the Christmas parties was more peaceful and laid-back, less frantic. For one of those, we focused on meeting characters, and had a great time in particular with the 7 Dwarfs. We also loved walking right onto the rides with little to no wait - since we were traveling with a little one, that meant Dumbo and the Teacups and the like, nothing big, but it was perfect.
With kids who enjoy Halloween, I don't think you could go wrong with either party. Travel at the time that makes the most sense for your family, and have fun!