Velcro - It's merely a matter of personal preference. If I break a buckle on a trip (and I have before), I'll still be able to close the bag, albeit in a slightly less waterproof fashion. I have a 5' tall 7 year old with very large feet who, on occasion, steps on things... And, I wanted the thicker Cuben bag for my quilt. Again, personal preference, not based on a scientific study.
Can't comment on the Cuben thickness of the other manufacturers.