Here's my feedback on the XS Spring Straps: It's nice to have a design that changes for different types of fins. But they have a problem in that there is a bit of plastic in them. The pin would still keep the fin straps somewhat secure if the plastic broke, but what's the point of paying for steel fin straps if they have plastic bit that can break?
Spring straps have the "steel does not break" advantage. But they have the "cannot adjust" disadvantage. If the "add frangible plastic bits" disadvantage is with both rubber straps and spring straps, then what is the advantage gained with spring straps? (If you use Jet Fins, you probably gave up on the "adjusting" part anyway.)
FWIW, I have a a pair of XS Scuba spring straps I will sell you cheap. Find the lowest online price, and subtract 1/3, and I will send them to you. Never been wet. I had to wait a month and a half to get a replacement part that came defective with them, though. I bought them for the "fits many fins" design. But that was before I played with them, and saw they used a plastic bit for support of the pins.
Now I am fairly convinced that the Piranha Dive Mfg design is the way to go (if you use side post fins), if you want to use spring straps. Their design puts the spring's hook directly on the side post.
I have never had to cancel a dive because of a broken strap (Because that's what save a dive kits are for.) But I have had to cancel a dive because of lost or broken plastic bits of the fin buckles. Usually the diver unbuckles the clip after the first dive, and the bit falls off the post. (Because not all fins use the same buckle design, you may not be able to substitute.) So I am not big on the idea of a spring strap that has plastic bits. They cannot go missing but they can break.
All research being piled in, I think the idea of using a bungie cord might be the best overall (if you want to avoid regular straps for whatever reason). But that still relies on the metal buckles from a jet fin. And I am not sure (because I have never tried it) how it would work with a traditional side post design fin. On a side post fin design, the Piranha "spring hooks directly to the side post" design woud be best, it seems. A side advantage would be the fact that you would not have the cutting edge that the XS Scuba design has.
I don't know anyone that has used spring straps long term, so I have no idea if that cutting edge does cause problems, but it sure looks like it would.