I've used both recently, here's my thoughts. Reaper's non-destructive editing is bloody priceless when compared to Audacity's. Plugins for reaper are... less

I've used both recently, here's my thoughts. Reaper's non-destructive editing is bloody priceless when compared to Audacity's. Plugins for reaper are... less well integrated into the application as a whole. Audacity makes for a great ACX mastering tool because of the open source, purpose made, plugins they have available. Reaper's ability to add SFX chains to tracks without modifying the track directly is pretty incredible. You can both live preview SFX changes, and change settings without having to undo between changes. I miss spectrograph editing when in Reaper. Word is there's an external plugin, but... see point 2.