Chelsea:
Hands down, if I had the choice, I'd go with the
Keys to the Kingdom Tour over the Mickey's Very Merry Christmas Party. Your hunch is correct: if it's your first visit in a while, the subtle nuances between the parades and the fireworks don't make it a better value over the Keys tour, especially since I don't think you and your boyfriend want to take advantage of the Christmas dance parties with the characters around Magic Kingdom....
The only thing that makes the party stand out to me from a regular day at the Magic Kingdom is the free hot chocolate (if you don't mind standing in line) and the very light snow flakes that fall on Main Street. But again, my guess is because you're from Ohio, you've seen snow... Because I'm a frequent visitor, I can appreciate the differences in the parade and the 180-degree fireworks, but I don't recommend it to every guest.
Your tour will be a once-in-a-lifetime experience. And my guess will be that you'll most enjoy going
under the Magic Kingdom to visit the utilidors, the underground passageways that cast members use to navigate a miniature city
under MK. I visited there for the first time last year and found it fascinating. This tour will be the perfect beginning to the park's theme for next year:
Let the Memories Begin!
Wishing y'all a magical vacation!
Whitney