Hi -- I have and use both, although my Selective Trout line was a prototype and the final product might have been tweaked. I used both on a Sage 590-4, a pretty fast rod. Here's what I'd say:
The Rio Grande is a great line, and even though it's a little heavier than AFTMA specs, it lands lightly. In fact, that's the most amazing thing about this line. It will load a faster rod without too much line out, but lays out the fly gently.
The Selective Trout line (at least the one I tried) lays out the line VERY gently. It probably has the softest delivery of any trout line I've used, and you can get a 5 weight line to land like a 3 weight. It didn't load my XP as well as the Rio Grande, but it was better for little dries. I also gave up something in distance. The head is longer, but somehow the Rio Grande shot farther. I think the Selective Trout line would be my first choice for a dry fly line on maybe a slightly softer rod, like a Winston, SLT, etc.
Finally, I really like Rio's other two trout lines, the Classic and Longcast and Windcutter. The Classic is a good general compromise that loaded my rod fine and delivered fine. I used it happily for a week on the Missouri with flies #18 and smaller. The Longcast is great for longer casts, but again I didn't find it to have a rough delivery at all. Caught lots of rainbows in the Delaware on the Longcast with 6X and never felt overgunned. In fact, on the Delaware, which sometimes requires longer casts than a typical trout stream for a wading fisherman, it's probably been my favorite. Actually the Rio Grand is heavier but has a finer, softer landing tip.
Almost too many choices, but they're all good. IF you have a fast rod, the Rio Grande might be preferable. If it's a softer rod for little dries, try the Selective trout.