We just returned from a 1/5-1/12 stay...and we arrived with the marathoners...(we was me, my college son home for winter break, and a single mom friend w/her college son (a friend of my son's) also home for winter break)...
The marathon was really awesome to watch (and you can't miss it as you go to your park destination Sat & Sun--there are two separate races--the 13 mile half marathon on Sat and the 26 mile full marathon on Sunday)
It made for the longest line I've ever seen at Magical Express...but it moved very efficiently...took 40 minutes before the ME bus started driving toward WDW, much longer than "usual" but nowhere near as bad as it looked like it would be from all those people in line!
We had Christmas decorations up almost everywhere til 1/7...after that we were "back to normal"
It wasn't crowded in the parks at all before lunch, and only "comfortable" in the afternoons/evenings (by comparison to the mornings--when it was positively empty each morning--it felt crowded; by comparison to our T-giving week trip, it was MUCH less crowded...longest wait line I saw (other than Soarin on Sat 1/6) was 30 minutes...
So, I think the marathon does make a big difference in terms of what you find in the way of lines, crowds, ADR availability, etc. And how much of that first week in Jan is still part of the Christmas holiday has an impact too...our last 3 days (1/9-1/12) it was Christmas-less and totally not crowded...
And it was all wonderful...
If I were choosing, I'd go w/a week that had a piece of each in it, like we just did...I can't begin to describe the goosebumps from watching the marathoners (as I was walking into the park; I wasn't part of the audience); it was so worth seeing...and it was neat seeing the tail-end of the holiday decorations, too (altho we'd seen them over T-giving and between that and our own Christmas at home, we were ready for Christmas to be over)...
Good luck w/your choice...