The major performance difference is that MLS was designed for military use on MOLLE webbing (or these MLS receiver plates), and QLS was designed for civilian LE with much higher retention standards to resist a physical struggle. QLS is also smaller.
If you see an L suffix on a QLS receiver, that means it has 2 little locking studs requiring the bottom of the receiver plate to be pivoted away prior to the forks being squeezed to release them. This is an option for extra security, but I don't prefer it. Takes the Q out of QLS since it then requires both hands to release it.
MLS is handy for MOLLE gear because the forks just slide/secure into it with no need for any receiver. This lets you move your holster/accessories easily between any suitable MOLLE space or any MLS receiver plate.
MLS 15 forks are for holsters, so they're longer than MLS 16 forks (which are for accessories).