Evga was the original manufacturer of Nvidia video cards and has never built anything but Nvidia cards just as ATI was the original manufacturer of AMD cards and builds only AMD cards. Both Nvidia and AMD licence technology to other manufacturers who build off the same core chip designs although manufacturing processes, components and quality may differ greatly.
I've always bought Evga. In most cases they offer a lifetime warranty. Without actually checking I'd be surprised if Zotac would warranty past a year.