Yeah, the QLS is the stronger of the two, as noted. The conventional wisdom is that MLS is more for military use -- direct MOLLE compatibility of the fork being one indicator -- and is designed to breakaway in the event of a snag hazard of some sort; think "paratrooper" or some such. QLS is stronger, in material and design, and is less likely to pop off in the event of a gun-grab rasslin' match, which is more in the LE realm.
Thing to note, likely not of any impact, in this case: On my UBL, when I switched from MLS to QLS, it put the gun about 3/8" higher, which was JUST enough to screw with my draw; overcome, with practice. No big deal on a shroud or other true drop-leg setup, since one can just adjust whatever the QLS/holster is mounted to. Just something to bear in mind of anybody decides to switch from MLS --> QLS, or vice-versa.
The MLS has longer forks, and while this might seem like a "DUH!" factor, it pays to be mindful mounting the fork into its receiver plate; it's not as self-guiding as it may appear. I've seen 3 folks who didn't align the fork quite right because they weren't looking. They just stuck it in place and shoved, and had one of the forks slide OVER the space it's supposed to go, and not into the locking chanel on the plate. Their gun/holster never fell off, but it's not ideal, and led to a tine deforming because it was left that way for quite some time. Once discovered, it was replaced on the basis that having been bent into a Silly Straw shape weakened it at the bends. Bottom-line lesson there is not so much one of material, than it is for one to conduct their equipment checks, and not just go through the motions.