Heretek Plus gives you an extra (free) weapon unlocked. A weapon that I suspect would otherwise be unavailable (similar to the Arc Scourge - which has an explicit installer explicitly for it).

I think installing only one of them is required to play Heretek - just choose whether you want the extra weapon in play or not.

That said, if you wanted to try the axe out and already installed the non-plus version, you can probably install the plus version over the top without a problem (hopefully). No idea if the reverse is true, guessing probably not?

I think that's all the difference, since the extra goodies are separate downloads anyway. If this is wrong hopefully someone will correct me.

But I'm sure you've figured all this out already, given you posted more than a month ago... ;)