I was there MLK weekend 2017 (actually Wed - Sun of the weekend). It was crowded with crowds building starting on Thursday night and being pretty darn crowded on Saturday. We didn't even realize it was MLK weekend until a couple of weeks before we went (our kids were in college (with a holiday break that ends early in January) or post-grad, so we are infinitely less aware of holidays that don't bring them home!).
We were there because my husband had a meeting in the Rosen Center by the Orlando Convention Center. Well, that place was packed when we stayed there Tues and Wed. (moved to Disney on Thursday). It turns out that the International Builders Show and the International Kitchen & Bath Show were at the OCC that Tues - Thurs. It was 1000's and 1000's of people. I'm sure that convention added to the crowds over MLK weekend once the shows were over. I know a lot of those people brought their families because I was shopping at the outlets while my husband was busy and I struck up a conversation with a clerk in a store and she was saying how busy they were the past couple of days because of that building show!!
I looked and those same builders' shows are there Jan 9 -11 2018. So, you're going to have that same effect on MLK weekend along with all of us northerners looking for a possible warm weekend in January, without taking days off school/work.
Also, airfare will be higher surrounding MLK weekend than the random days after the New Year.
So, to shorten my answer - I might pick Marathon weekend. There will be traffic implications from what I understand on the day of the race - so buses will be impacted. But, I think general crowds will be lower than MLK.