I've flown a Rio 2 but none of the others. I think the Rio 2 is very similar to the Sting - about 60% double surface or so, light, has a VG which improves the sink rate a bit but not as dramatically as on a higher-performance glider like at U2. Nice easy handling and landing, and very easy to set up (no tip wands - I hate tip wands!)
If you're into dune gooning though I'd have a look at the Avian Fly - it uses the same airframe as the Rio 2 but has a single surface sail, so it's close on performance but is lighter and more nimble. It has a higher aspect ratio than most other SS gliders and I think rather better performance, although I've never done a side-by-side test. It has nice light pitch which makes it more fun to whip around for an experienced pilot, and you can get it with a VG. It's possibly similar in performance to a Northwing Freedom, but I've never flown one.
I used to work at Avian's factory in 2012-2013 and almost sold my U2 for a Fly - I used to love flying that thing, I always had a blast on it. There are some photos of me flying it on Avian's Facebook page here - I took one of the factory models fitted with a Supine control frame to Ager which was great fun!
https://www.facebook.com/pg/AvianHangGl ... tab=albums
https://www.facebook.com/AvianHangGlide ... 623995356/
Hope this helps