Power.

53 lb kb x 3 foot swing=159 foot pounds
159 ft/lbs/2 seconds rep=79.5 ft/lbs per second

72lb kb x 3 foot swing=216 foot pounds
216 ft/lbs/3 seconds rep= 72 ft/lbs per second


I'm no physicist but I think those numbers are right. What I discovered when we were looking at trying to measure power and watt output of the kb swing and snatch 18 months ago was that many times the lighter bell gave power numbers that were higher than the heavier bell, because the heavier bell moved slower.
And doing multiple low rep sets, focusing on TRYING to keep the speed up( ala WSB) really activates some serious fast twitch fibers, especially as fatigue sets in.

Even if the number was even the heavier bell exposes one to more injuries.Moving a lighter bell faster can produce more force, and more power, than moving a heavier bell slower. The argument should be about whether the kb is as good of a power tool as it is a strength endurnace tool.
Those that say it is too light to be a power tool just dont understand the force velocity curve.The bell HAS to be light enough to be able to be expressed powerfully.No it is not the ultimate demonstration of power BUT it is a tool and group of special exercises that will allow one to become more powerful through it's use and the adaptation to it.