I think they're meant for totally different applications. The Zendrive is smooth like a Dumble, and the Fetto sounds like a cranked Marshall. I haven't played either, but that seems to be the consensus.
I have a Fetto Deluixe but not a Zendrive. The Fetto Deluxe has a ton of flexibility and can serve as a very good overdrive, but as the poster above said, it's bread and butter is as a distortion in the Marshall vein. Very ferocious.