Film DCI4k or UHD for exporting DCI-Scope but watching on UHD screen
I researched a bit more on my own and it looks like there is a diffrence but no big one.

While a native image will look better, the image quality isn't much diffrent when "downscaling" to UHD.

It might be noticable on a high quality projector screen, but on desktop and espacially on mobile screens it shouldn't be a problem.