We were in MK on 12/29 last year and it was . . . not fun. First, we were staying at BWV and the traffic getting over to MK was a nightmare. It took at least 30 minutes longer than it typically would (so I imagine trying to drive in from offsite is even worse). Then, the crowds. We had FP+ for our must-do's and that was really all we could get done as the lines were out of control. In fact, simply walking from one spot to the next was really uncomfortable. At one point, we went to get my DS's face painted just because no one else was there and it was the least crowded, least chaotic place in Tomorrowland. I can't even count the amount of times we almost got run over by scooters that day.
We ended up leaving around 1 pm because we couldn't take it anymore. We were staying at BWV and had multi-day park hoppers, so I really didn't mind leaving as we then went to EP in the evening (which was a little more manageable) and we spent time doing a tour and doing a lot of non-park activities. So, all in all, it was a good vacation, but not a great time to visit the parks. I would do it again, but I wouldn't even go to MK over Christmas week. EP was doable if we did WS in the morning and FW in the evening while most of the crowds were lining up for Candlelight Processional. DHS was surprisingly not too crowded, but I wouldn't spend over $500 to go to DHS, AK or EP for one day, especially with the crowds.
I would stick with Universal on this trip!