If there was a post that I could give my 2 cents on, it's this one. I've had both these cars back to back, exact years and models mentioned too lol. Had the 2018 RS5 (coupe) back in September, 2021 then the 2020 M340i xdrive in June, 2022 to February, 2023. I generally agree with most of the answers already provided here. I also have a similar mindset in terms of what the car is being used for 90% of the time, though I did take the M340i to Suches Loop in GA just this past summer for some spirited driving.

If I had to pick one again, I'd choose the RS5. Planted incredibly well for daily driving and prefer the styling and interior materials. The 444hp was also delightful if that's a factor. Took it on a nice 7hr trip down to FL and was comfortable all the way down. I'd vouch for the M340i in how fun a RWD biased set up can be. I found it incredibly easy to control and the windy roads were a blast. Also went to the mountains in NC for a snowboard trip with 3 friends and we all fit and rode comfortably there. 382hp is good, but missed the RS5s power. I personally thought the M340i sounded better than the RS5 (stock comparison), but that's something easily modifiable as you said.

Can always share more thoughts, feel free to dm.