Having been a massive Who fan when I was younger and now a massive Zep fan, I'm in quite a good place to be able to analyse the two. I think Zep are much better musicians, generally. Their songs are a hell of a lot more complex and intricate and well written. The Who are more rough and ready, which was always their style I guess. They both kick ass live. I think something like Who's Next is most similar to some of Zeppelin's work....that was a pretty well written album.
Also, the intro of Song Remains The Same really reminds me of something Who-esque.
Similarities: Blonde curly haired singer who can rock out...wild drummer who knows how to pound the skins....intricate, excellent bass player holding the band together.
I'm not even gonna go down the road of comparing Townshend to Page because they're not even in the same league. (As in, JP is a million times better).
Also, a lot of people compare Bonzo and Moon...whilst I think they're both very excellent and unique drummers, I've got to say that Bonzo does cut it slightly higher for me, simply because he can do everything that Moon can on only one bass drum and three toms...whereas Moon had double bass drums and a shit load of toms. So, to me that means he's a better drummer. It's more impressive.