I've been using webbing w/ double rings or cinch buckles for a while and just recently bought a WL Lite Owl that came equipped with Whoopies.
I loved the cinch buckle system for simplicity and might replace the whoopies witht that.
One downside to whoopies is that any weight on the constriction part of the sling keeps you from adjusting. Even the weight of the hammock itself keeps you from being able to adjust the sling. It's no big deal--you have to tuck the hammock up under your arm to take the weight off the sling to adjust it, but it's a little more involved than simply pulling the webbing tight with cinch buckles.