I think influence versus importance is a very relevant distinction.
Bowie has a great quote about the Velvet Underground. Something like, "only 1,000 people bought the first Velvet Underground record, but every single one of them became a musician." I think this keys into the importance of the Velvet Underground, but...
I think its impossible to ignore the fact that influence is directly related to the people impacted by the music. And Zeppelin, no matter how you slice it - and as it was mentioned previously - reached more people. Regardless of whether or not they were ripping off old blues songs, they got it to people that hadn't heard them before. The volume of people that heard those Led Zep. records, and went "wow, I need to know more about this" or "wow, I need to learn how to play the guitar," its impossible to estimate the influence that they had. But by simply reaching a larger audience, I think its likely that they had more influence on the music listening public.
If people have seen "Dig!" there is a great quote that Courtney Taylor or one of the Dandys makes about Anton Newcombe. He says something along the lines of "its impossible to start a music revolution if you remain underground." Certainly not to say that the Velvet Underground remained an unknown band, but I think that's a good point of reference here. Led Zeppelin flat out reached more people, and by virtue of that, I'd have to say that they were more influential. Yes, it is a volume issue, but that also translates into different sounds: look at grunge, modern rock, metal, and so forth were all influenced by Zeppelin in some big ways.
But more important is a different question altogether. Then you start factoring in the questions of originality - and that's where you probably have a stronger argument for the Velvet Underground.