I had the 800 for a while but really wasn't happy with the long MFD and the f/11. IQ was pretty good and I am sure it would have improved as I got used to it more. I liked the handling and size weight a lot.
Now I have the 100-500 and it is a real winner. It is lighter than my 100-400ii, however I don't have the RF teleconverter yet so can't say how well it works. I have always had mixed results with the 1.4TC on the 100-[\400 although the TC's work really well in my hands with my 300 2.8. Maybe it's me I don't know, but when I use the 100-400 with the TC on either my 5DIV or the new R5 it sometimes works really well and other times maybe not so well. The RF 100-500 is pretty great so far. I will admit that my time with it has been a bit limited so far. I took it to the Stick Marsh last Saturday and it was a joy to use. The images looked great when I got home and examined them. And the MFD is pretty amazing although I haven't really wrung that out yet.
PS I got mine from Canon USA when everyone else was out of stock. Go figure. I went on the site just for the heck of it and was startled to see it listed as in stock.
The 800mm and 100-500 with 1.4x are an interesting set to compare. Both have a 70 mm diam front element. So light gathering is the same. If Canon had a 1.6x TC and you put it on the zoom, they would both be 800mm f/11.
I'm thinking the 100-500 would be the winner if the critters were closer, enabling shooting with the bare lens. I think there are lots of times you could have such scenarios in Florida. In most other places I have been to you need more reach, so you really need around 700-800mm, and are often cropping those. With the 1.4x on the zoom, I think the rigs would be similar in IQ. The 100-500mm has much closer focusing, has zoom versatility, but is heavier and costs a lot more. Tough decision.
Let us know your impressions as you get more experience.