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.