I have not used the RF 100-400 enough to form that kind of opinion. My initial impression is that both are good enough for my purposes. I’ve had the 800 long enough and shot in a wide variety of situations to confirm my initial impression. They both pair well with my R6 and fit in a small backpack for a lightweight carry. I won’t speculate on results with an R5 - see @AlanF
That’s a bit of an apples to oranges comparison anyway. These lenses serve different focal lengths/purposes for me. I am generally not a pixel peeper so test charts will not matter much to me. Comparing a cropped 400mm photo to an 800mm photo doesn’t make a lot of sense to me. I don’t plan to get the RF 2X extender so a head to head comparison is not important either. I won’t bother comparing the RF 100-400 with the EF 100-400 L II, many others will do that and confirm what we already expect. Maybe someone will compare an EF 800 with the RF 800 someday. They can all take great photos.
I bought the R6 and these lenses for size/weight considerations. The 1DXII and L glass are getting a bit heavy to lug around all of the time as I get older. Most of my everyday shooting had switched to the smaller/lighter M series, but it lacks telephoto options. The R6, RF100-400, and RF800/RF1.4x addresses this need much better than adapting EF lenses to an M body and gives me more capabilities than I had before. And it costs a lot less the the EF 600 L that I considered a few years ago, but never pulled the trigger due to size/weight.
I still use my EF glass on the R6. I shot with the EF 11-24 earlier this week. The 1DXII is my backup body for now while I wait to see what new R bodies are on the horizon.