Oh that is a hard one! I know a lot of people are very superstitious about flying on Sept 11 so you need to decide if you’re uncomfortable with that.
For me, I’d go on the 11th, have a nice dinner outside of the parks (resort hop or DTD) and maybe catch some fireworks or turn in early and hit it hard the next day. At the price of POP, I’d pay it to be there and ready to go the next morning, hopefully rested. Touring all day after an early flight to get there in time to enjoy it can be exhausting. It doesn’t bother me but DH prefers a little slower pace.