Well, these dates will most likely all be the same. Its summer now. People come and go daily. With the concert on the 18th, I would assume that IOA will be less busy towards the evening hours. We are going to be there June 15-22, so I am just prepared for the crowds. We are staying onsite, so we will not have to wait in many lines, but it will still be busy. If you are really flexible, wait and see what the weather is looking like. Granted, its summer so rain will almost always happen daily, but maybe it will not be as hot on the 2nd set of dates than it is on the first.
Of course, if you really want to see the concert, then go with the 18-20.