I'm using Ardour on a daily basis and looked into Reaper for a while. Reaper has some very nice stuff IMO:
- the js plugins and the ease of creating those
- built in oversampling for plugins who don't support it but should
- probably more stuff that i can't recall right now
I think Reaper is a great DAW, however (ymmv) i find it more complicated in almost every aspect. I have used Cubase, FL Studio, Nuendo, Qtraktor, Ableton and bitwig in the past and started using Ardour a year ago. Everything was very easy to understand, the only time i had to read the manual was when I wanted to integrate a midi controller that had no bindings avaible. The Mixer is great. It's intuitive, has a good UI and many features like pre and post fader plugins. Routing is super easy for me.
The only thing that's a bit clunky is programming midi. At least for me.
Reaper on the other did not had such a welcoming beginner expierence for me. I looked into it a few months ago despite being very satisfied with Ardour (because of fomo i guess), and it was not that easy to set up a project in the way i wanted to. Maybe i was to much into the Ardour workflow by then, dunno. But my final verdict was "Looks nice but I pass".
Hope this is of any help to you.