Zeph for sure. Kazakus can lose you games where you need that one specific potion ingredient out of two tries and don't get it. If zephyrs whiffs, it's because you failed to set it up properly
Except Zephrys isn't a human and can't always give you the best answer even though you can set him up well. I'd go for potion RNG anyday over hoping that Zephrys gives me a Poly or Hex at 6 mana against a Big Priest...
He doesn't always give you the best card, but he always gives you exactly what you expect him to if you know him well enough and that information is powerful. If you know he is not going to give what you want on the given turn, then you don't play him. With Kazakus, you don't know if he has what you want until you invest 4 mana
If you play Zephrys against big priest, you should that know he won't give you a transform effect to begin with because he can't see all the res cards in the opponent's hand. Expecting him to give you one of these and wasting 2 mana/the battlecry to check the box is considered a whiff in my books. Transforming Big Priest things isn't in the scope of what he can do, much like Kazakus has other things out of his scope too