The ability of the defensive variant sounds strong but I think in multiplayer people will know to only hit them down to a low number and then leave them there until you cleared out the other Hive stacks. So when the Reavers time to cheat death comes the battle will be lost already anyway.
The offensive variant seems to be the better one, but of course we'll have to try it out.
Very broken card in Hearthstone, even when it goes off just once. Getting one extra turn is pretty strong. Even if the unit that's left isn't great, that's still an extra chance to cast a spell or retreat. Granted, the Reaver is less powerful than Ice Block because you can kill it twice before it gets to act, but the utility is there. I think the other upgrade is better for an endgame army, but this certainly has its utility for scout heroes and such.