I've never been in January, when I hear it's light, but from my experience, September is the lightest time, crowd-wise, I've ever seen. By Wednesday after Labor Day, all the kids will be back at school, and few companies schedule conferences and things in September out of respect for their employees who have children just back in school. I was just looking at my video from last September and we kept commenting over and over that the parks, Epcot in particular, were ghost towns. Notable exception: MK on Saturday's. All bets are off there, the place gets jammed.
The hours are lower, but you can get so much done. We found by late afternoon each day, we'd done more already than we normally get done during June visits. In particular. Wednesday was an incredible day to go to MK. Splash Mountain was a walkon, three times. Space Mountain, walkon. Pirates was a walkon. The only wait we had all day was Big Thunder, and that was five minutes. They had the fast pass turned off at the Haunted Mansion due to short lines. Just incredible.
And since we'd always disliked the fifty degree nights we'd run into in late October to early November, we were stunned to find it's still high 80's to low 90's. Not the gripping heat of July, but at least you know you're in Florida. We went to Typhoon Lagoon and had no wait for anything, and spots to park it for the day were plentiful.
Hope that all helps. The short version of this message: You've made a wise decision. Go to MK the Wednesday you get there.
Pat