serpa lock? do you mean the retention devise on the serpa holster? if so I'll tell you about my experience with it.
bought one a while ago for my M9, mounted it on the chest of my IBA, lasted less than a week, the spring in the retention thingy broke, so I bought a tactical taylor holster, however a ton of the other guys in my unit have the serpa and have had no problems with it, manly using it on a drop leg rig.with a few chest rigs. I would suggest going with the serpa that fully covers the slide, keeps the gun cleaner longer.
hope that helps a little