I don't have an exact apples to apples comparison, but maybe close. I have the 3-15 x 44 HD5 and a 3-18x50 VX6, not the HD version. In talking to someone on this board, he tells me that the glass between his HD and non HD version are pretty much unnoticable. So that said, I tested the VX6 and the HD5 at full light and low light on an eye chart at 130 yards. My son also looked and gave his input. In both cases, both low and full light, we could both see about 1.5 lines farther down through the VX6 than on the HD5. This held true all the way until dark....though we could see fewer and fewer lines as the light went down, the gap between them remained pretty consistent. BTW, my son could consistently read about 1 line farther than I could, that also remained consistent.
As much as I like my HD5, I moved it down in the pecking order, putting my VX6 on my most used rifle.
As mentioned, if you are shooting fairly long, the zero stop system on the HD5 is not so great. One turn, about 17 MOA, unless you remove the stop. I do like the locking turrets on the lockplex, and when I bought it, it was one of a very few that had this feature. It also has a thicker plex reticle that is easier to pick up on the target in lower light than on the Leupy.
However, the VX6 HD (and the 5 as well I believe) now has 2 turn lock turrets, and I had them retro fitted to my VX6 for very little money. Great system. I would recommend a firedot or illuminated reticle if you can swing it. HTH.