I have only lived in Kansas since 2007, but this is the worst summer I've seen. It has been triple digits for almost two weeks and I find myself praying for fall, as well. My horses are living on about 10 acres. We have 40, but we have been so dry, the creek is dry and the only water is on the 10 acres. I worry they won't come in to drink. If the lead mare doesn't bring them in, they won't come. For now, they are close to water, and have access to a lot of shade and their shelter. I installed a misting system in our 50 foot loafing shed, and they do spend their resting time in the shelter. It wasn't that expensive, doesn't use a heck of a lot of water, and drops the temp up to 30 degrees. Very simple to install, and horses love it!
Regarding in or out, if they have a choice, I find that a bit better. Personally, I'd rather be outside where the air is circulating, there is shade and cool water. Fly preditors keep the flies down, and you can use fly masks to keep their eyes protected. It's almost fall!