Hi Max,
5/16 is awfully small for a furling line IMHO, 3/8 is far easier on the hands... Even on my relatively small staysail, I wouldn't want to go smaller than 3/8...
You want a line that's easy on the hands and easy to grip, non-hockling and free-running, and without any tendency to acquire a "set" when on the furling drum... A braid like Regatta Braid does all this pretty nicely, although you have to make sure your decks and leads are free of anything the braid might snag on, that's the only real drawback to such a line...
I've been using NER's Salsa for furling lines and my mainsheet for several years now, it's very nice stuff for such an application... A rope like Samson's Apex would also be nice, probably a bit quicker drying, always a nice feature in a furling line... I've recently switched my genoa furling line to Samson's MLX, a very nice line in a sexy Euro-style sort of way... But, as you know, I'm all about what looks the coolest, in the end... (grin) It's a much smoother line, a bit slippery through the hands and harder to grip, but on the other hand lays very nicely and neatly on the furling drum...
I'd just try to get my hands on whatever you're thinking of first, and make your decision based upon that...
The line on the right is the Salsa on my staysail furler - nice and "grippy", even when wet...
best regards,
Jon