We went in July, tried an early day and quickly came to the conclusion that way too many people use the early entry. Everything, buses, etc. were very crowded. Parks were jammed.
We then tried the day after strategy ( MK early day is Sunday...go to MK on Monday). It worked much better. Lines didn't get big (over 20 minute waits) until around 11 am, even for the big attractions. We'll never use early entry again.
We felt that is was best for us to get to the park at opening (not on an early day), stay till around 12-1, go back to the hotel for a swim, rest, etc. and come back between 4-5. Surprisingly, there was a slow period at the parks from around 5-7 when lines for the rides were no more than 15 minutes.
I would try a day of both and experiment. Crowds and conditions in September will be different than July, but this worked for us at a very crowded time of year.
Good Luck and have a great time!