You could always get one of each, pick the one you like shooting most, and let the other keep the safe warm - j/k.
If you already reload .223, then the .223 VEPR might make sense, lots of ammo, and more types of ammo is available for it. Drop in a Red Star trigger group, and it will give your Oly a run for the money. I'm not saying it will ever be as accurate as an AR, but my VEPR K does a great job in 3-gun matches.
As everyone else has already pointed out, it is easier to find mags, accessories for the 7.62x39.
The 7.62x39 will have more knock-down power for those stubborn bowling pins, but you will be able to reach the ones farther out better with the .223
I have both, bought the 7.62x39 VEPR II first, and then the .223 VEPR K.
If you are looking for something with a little more thump to it, get the 7.62x39. But you can't go wrong with either one.