There seems to be way more 3rd party plugins. Reaper has quite an extensive API while Ardour is open source. There was quite a long discussion about this on the Ardour forum. It seems that the Reaper API and general extensibility gained more traction than Ardour being open source, as counter intuitive as that seems. Having used both pieces of software a reasonable amount, Reaper has a slight edge in terms of usability and customisation of the layout and look and feel. In particular the midi editing is smoother. That said, Ardour is a fantastic piece of software and a great open source success.
LV2 plugins aren't supported in Reaper but are in Ardour, this makes quite a difference in Linux. In my opinion the anywhere to anywhere routing is better in Ardour. That said, I moved from Ardour to Reaper and prefer Reaper.
I've tried using it but it crashes too often and is generally too buggy to use. It seems to be in an early stage of development so this is to be expected. The UI and general workflow look very promising though, in particular the piano roll. It has a long way to go to match the maturity and features of Ardour, but the UI design that it's aiming for will make it more suitable for people making EDM.