We went the first week of June last year, stayed offsite, and went to both Disney and USO. After reading about Disney summer crowds on here, I prepared myself for the worst. But, it wasn't that bad, we were pleasantly surprised! The humidity stunk, though! Blah! We had a great time in spite of it, although I will say I am looking forward to going in early spring and hoping for less humidity!
This coming April we are staying onsite and doing a split stay between WDW and USO. It would have been a bit cheaper for us to stay put at Disney, but we wanted to be onsite at US to be able to go back and forth to the parks at leisure, and also to have the Express Pass, as we are going during spring break season. I have read on the US board that discounted rates have not come out for the resorts yet, so I am hoping for a lower price, but if not...it will be ok (that's my mantra for this vacation, lol).
My daughter is a huge Potterhead and LOVED the HP area. It really is amazing...the detail they put into it, oh man, it is so immersive. Your daughter will really enjoy it. Even my son, who has never seen the movies, was in awe.