Its ususally very very very hot in disney in the summer. Starting in June-October even. I say tank tops! Even a simple tshirt may make you that much hotter. Just wear a higher level of sunscreen and you will be all set
I don't think I could wear tanks to the parks, just the thought of the sun beating down on my shoulders and neck makes me cringe. I just don't like getting that much sun, and personally I think I'm cooler if I wear light colored, light weight t-shirts.