The Pro version has been Roon-ready certified since the June 30th FW update, per their web site. I have been using the Pro in my Roon based system for a couple of months now.The other difference, currently, is that the Pro is certified Roon-ready, and while the Plus is in the queue for that certification, there's no known ETA. That will make a difference to a minority of people, but to them, it'll be a big difference (i.e., a show-stopper).
Otherwise, if you're feeding your favorite DAC, unless they'll cost you the the same, there's no compelling reason to get a Plus over a Pro.
https://www.blog.wiimhome.com/post/wiim-pro-is-roon-ready-certified