Will it be that much worse in Orlando than Anaheim?
In a word? Yes. And many will tell you that no, they would NEVER go that time. But obviously lots of people do go. We're one of them, heading in 2 weeks.
The heat will be HORRIBLE. Your highs will be around 90-95 every day - and that's before the humidity. It will feel well over 100 every day. You'll be going during one of the rainiest times of year, there will be thunderstorms and likely ride shut downs due to storms at some point, every day (from what I've been told). You will be soaked in sweat, and you will need to tour very differently at DW than DL. Mornings for rope drop, then take a break for the afternoon and go back in the evening when it gets cooler, or just do mornings or evenings. But remember that "cooler" is a relative term, if you look at the hourly forecasts, you'll notice that there's typically not much difference in temperature between morning - evening, though the humidity is lower in the evening.
Compare that to
Disneyland, where the average high will be only 75-80, with very little humidity, and likely not a drop of rain.
Since you have the choice, I would take Disneyland
hands down, over WDW, if that is the only time you can go. We are going in two weeks, and I know it's going to be really rough, heat-wise. I wanted to go to Disneyland, for this very reason, but DH is not a fan of flying and thus, California just wasn't going to happen. That said, if you can find another time to go to WDW, then I would wait and try and go there at a different time - November, early December, or anytime between mid-January to end of March.