I don' t really have a comment toward double or single calipers. I have used both but prefer the double. Someone has mentioned that you don' t know if one of the calipers open before the other on a double. And that may be true in some cases. However, the consistancy of the caliper is of equal importance. curently, I am shooting a rope release off the string and am happy with it.
As far as the velcro vs strap. If I have a choice, I' d go with a strap. Opening and closing velcro in the woods absolutely drives me nutts! I always try to stay velcro free in the woods.