I had the same concerns w/ our coming December trip, (Dec. 15-19th) as I know the parks will be packed. Our 2017 December trip, although earlier in the month than this years trip, was not too bad and we were on the Harbor side, but there were times when we were re-entering the park in the evening after a break that wait times were at least 20-30 minutes +, sometimes they had a no bag line, but it seems from what I read they aren't consisent. For our upcoming trip I took that into consideration and we will be staying nearer the DTD side as I've read the lines on Harbor have gotten worse, sometimes all the way out to the street and I can't imagine they won't be at their worst on the days we are there. People might say, "oh just walk down to the DTD entrance if its too packed"...but, that's a fair amount of additional walking, especially as once you enter DTD you have to walk the same distance again to enter the parks and I'd personally rather save my feet. I do believe though that at the January time you are going the parks should not be Christmas crowd busy, although maybe the 18th and 19th (near MLK holiday on Monday) would most likely be more crowded. If I was going at that time I think I would stay where I wanted to without much thought to which side, just knowing Friday and Saturday would likely be the most busy.