Just did a quick Christmastime trip with my 73 and 75 year old parents. It wasn't their first time, but it was all of our first time there for Christmastime.
We did
MVMCP on the evening of our arrival to WDW (we'd spent the night in the MCO Hyatt and hopped on the first Magical Express the following morning). We'd already been to Epcot, explored the resort, and then headed MVMCP. It was chock full of Christmastime wonderment, and my mother was like a kid in a candy shop, but the festive events were crowded and required staking a spot and waiting if you wanted ideal views. My dad gave up before Holiday Wishes and went back to the resort. Mom and I stuck it out and left around 11pm after the fireworks and a quick stop to meet Mickey.
Honestly, I think you'd get more bang for your buck with a full day at MK. You can still see the castle lighting, the tree and decorations. I think the trolley show is Christmasy that time of year, and there is always "Jingle" Cruise too. Plus, if you're doing Hollywood Studios, you can stick around for Jingle Bell, Jingle Bam, which was honestly one of our favorite things from our trip. My folks loved it!
To sum up...I'd say full MK day over MVMCP. I love early mornings at MK though. And there just seems to always be something going on there throughout the day. On our random "Let's go to MK" mid-morning day, we got last minute fastpasses to three favorite rides one right after the other, saw that Move It Shake It Dance Party parade thing, then saw the regular castle show (Mickey's Royal Friendship Faire), and got PhotoPass pics done in front of the castle and the Christmas tree. All in just a couple of hours. We would have stayed longer and meandered around, but it was hot and Mom and I wanted to hit the pool before heading to Epcot for dinner and Candlelight Processional. But our favorite Magic Kingdom times are always the ones where we're just wandering and taking in everything as it comes.
Sorry for the ramble. The trip is still fresh and I'm having fun remembering it!