I have used the SA Nymph line and studied the Rio Nymph. The SA line is a great longbelly wf line with a short front taper to assist in powerful turnover. I think it is actually the best line out there for roll casting and one handed spey casting. Its only drawback is that it is quite bulky and will occupy as much space on a reel as a gpx of the next size up.

The Rio line has a taper which is more similar to the SA XXD, with the exception of the front taper. The front taper on the line is only 3.5' and has a convex bullet shape to it. I would expect this to REALLY turn over powerfully. The welded loop will also help float the tip, something the SA nymph line did pretty well, but not perfectly and permanently.