Maybe it's because I'm still relatively new, but I couldn't really tell the diff and I shot both side by side. So if it were me, I would buy the L cause it looks badass. But factoring in the cost, new holster, and MRD that I would add, it's way down on my list of things to buy.
What differences were you looking for or expecting? Felt recoil? Accuracy? I haven't shot the VP9 much in general, but I don't think I'd appreciate much of a difference either.
I haven't really shot my PPQ 4" and 5" side-by-side much. I bought the 5" mostly for competition, but I've also shot the 5" model in training courses as well though. I'd probably have to try again and maybe shoot something like a mag with one and a mag with the other. I think the 4" model might feel a little snappier, but probably not super noticeable. I didn't think of the holster cost much because the 5" was pretty much mainly for competition and the 4" for training and CCW.