USSOCOM tested KeyMod and M-LOK some time ago and the results are now available for public consumption. The second to last presentation slide is the summary which showed that given proper mounting, M-LOK does better in drop test and failure load
Soldier Systems Daily article
http://soldiersystems.net/2017/05/05/details-on-the-ussocom-sponsored-keymod-vs-m-lok-test-conducted-at-nswc-crane/
The file from briefing can be found at the bottom of the artivle.
From: soldier Systems Daily