Yes July is significantly hotter.
Actually there have been a few times I can recall that in late May it was actually on the chilly side with all day rain but tha'ts not usually the case.
Nice thing about July is that you know what you're getting.
Hot, humid, and usually an afternoon quick passing but STRONG thunderstorm.
Also if you're considering late May be aware of when Memorial Day weekend is "the unofficial kick off to summer" when many schools are out for summer and many others that still go to school until late June get a long weekend.
And in the last couple of years crowds have really shifted from July to October so July tends to be a little bit of a slower time now than it used to be.
Overall I'd recommend July instead of May.