The difference isn't as evident when, say, you have your hands at arm's length, but up close, Nein! is visibly darker. It's also a true neutral-toned grey, whereas Power Clutch is more green-based. Formula-wise, both polishes are excellent and require two coats for opacity.
Do you need one if you have the other? Well I guess it depends on how much you like greys. They are different, although not dramatically so. But if you're not a big grey collector, you'd probably be fne just getting one of these. Personally I'd give a slight edge to the OPI because I love what a deep grey it is, but that's more a matter of preference than anything being "wrong" with the Essie. After all, these are probably my two favorite greys in my whole collection! What's yours?