First of all, best I can tell, over thirty trips isn't enough to do Epcot. Just an opinion
I go each June and occasionally in early August on Business. My wife and I take pleasure trips in the fall. I can't speak for January, although I've heard it's great. But in late September and early December, the parks are barren. Low lines, low costs, great stuff. The hours are shorter, but you can get so much more done it doesn't matter. I think there's an extra three hours at MK in summer, and the difference is much smaller at MGM and Epcot. That extra time is easily made up in not standing in lines or behind slow moving crows.
The difference between the two times: we loved September because it was still hot. Early December, to me, didn't feel like Florida. I know folks who prefer December for the same reason. Depends on your tastes.
FYI, we've been in late October, and it's about between the two, although the crowds were a wee bit higher, but not much.
But do Epcot in a day? Oh, man. The only thing you can do in Epcot in a day is develop a desire to move to Orlando!
Pat