I've been many times in June (last in 2016) and once over Thanksgiving (the Tuesday before through the Tuesday after in 2015) I would call the crowds pretty similar based on my experience. We spent Wednesday at DHS and that park/day felt the most crowded, but it could be also due to it being the park with the lowest number of attractions. The rest of the week felt pretty much similar to the June crowds we're used to, with much nicer weather.
I know the MK will be decorated by the first Christmas party. The other parks/resorts I can't say for sure. I know they work on them through November, but all will be done by Thanksgiving week. Epcot will start it's Christmas activities that Friday.
My family's favorite trip was probably the one over Thanksgiving and I had planned to do it again this year. But our High School is planning a Disney trip in June 2018 and my sons want to go. So we're skipping this year and doing a longer Disney/maybe Universal/Graduation trip ....so back to June we go.
regards
