Well this is the thing. I've had 2 knock off sets that absolutely don't hold tight when the rig is in motion. The shoulder straps come loose and kill the purpose of having it in the first place. I recently found out that the better sets have a spring inside the buckle that keep the tension held even when you're bouncing up and down. I've managed to 'fix' my current harness by adding some cable ties, cutting off any excess once fully pulled, to act as a pseudo spring. This stops the buckle bouncing loose in motion, but also makes the harness annoying to use once you're getting out.
So yeah, no motion? Any set will probably do. With motion? You really do need a set that is going to hold the tension on the straps. This is basically the only difference that I can see in them, but it's one that's pretty much a necessity.