If rhino is made from a higher grade of HMPE, then it likely will have (slightly)less stretch, pending all else is the same (over-all diameter, build quality, etc.). I've never been able to get an answer as to what specific materials are in their products so I can't make a comparison that way. I don't care to use their products after the way I was treated by a CSR.
Knowing the grade would give a much better idea of how the materials would perform if they did. It's like someone asking a bowyer "what's in your bow" and the answer being "wood". Doesn't tell you a whole lot.
There are only three major bowstring manufacturers in the world at this time. BCY, Brownell, and Majesty (Japan).
BCY dominates the market. Although Brownell owns the "Fast Flight" name, the founders of BCY invented it when they worked at Brownell. BCY is also responsible for the first blended bowstring material, and they are the first and only to be authorized to use the newest and highest grade of Dyneema, SK90. Point being, BCY is leading the way.
Field testing is just that. It's saying a bow shoots X fps because that's what your eyes or some homemade equipment told you--opinion. Field testing won't hold up in court.
Controlled testing is using a Hooter Shooter and a properly calibrated chronograph.
I have 2 decades worth of experience building strings as a business, 15 or more of that with Dynaflight '97. I've built literally thousands of Dynaflight '97 strings. That would edge out 4 years shooting two different strings, at least for me.
Mike, I agree--it boils down to what someone likes and/or believes in.
If stretch/creep is a problem, materials like 452X and BCY-X solve it. If you want to stick with a 100% HMPE product, 8190F and 8190 Universal are made with the highest grade available.