Tough one....both fathered a new breed of music and both brought a screeching halt to the existing mold. I think time will ultimately tell, but thus far I'd give it to Zeppelin.
Still, in a strange way, Nirvana shutdown the genre of music that Zeppelin primarily influenced while at the same time Nirvana was influenced by Zeppelin.
Yeah, gotta give to Zeppelin.....I'm no music expert, but that's how I see it.