As an owner of both the VP9 and the VP9sk, I love them both. They are both excellent shooters, and have not had any fails with either of them. I'm 100% honest when I say that I will not carry a weapon that I have to depend on. The SK is definitely a better carry gun, due to it's shorter measurements, and it really honestly gives up ZERO performance due to it's smaller size. The only noticeable difference, is that you have to shift your grip when changing mags. Because of the shorter grip length to avoid pinching your palm when inserting a mag. But you're gonna have to do that with ANY 2 finger grip frame.
Now, I said before there's no sacrifice in performance. Quite the opposite, actually. The SK is a softer shooter, and I shoot tighter groups when I'm trying for groups, with the SK than I do with the regular sized. Most likely due to the dual recoil spring in the SK vs the single recoil assembly in the FS. (fullsized)
9mm is also less snappy, so as a carry gun, you are back on target faster for a followup shot. Which is why I moved from 40 back to 9mil.
That said however... so far, you're mostly .40. Keeping your ammo consecutive would be a smart move. It also means interchangeabilty throughout all your guns. ESPECIALLY if these ***kers decide to make us start doing background checks for ammo! There is also the fact that once the VP40's sell out, that's it. That by itself in my mind says to get one while I still can. Assuming that our gov't still allows us to, you can pick up the SK at a later time if you decide to do so.