Answer by Robert Scoble:
I've spent time with both several times. I find Bill Gates is just an amazing intellect, and I don't think it's fair to compare ANYONE to him. Zuckerberg is also an amazing intellect, but it's different and we haven't had as many opportunities to study him the way we have had with Gates.
Bill Gates has a photographic memory. One time he said something on stage word-for-word that I had told him six months prior. Many people I know have these experiences with Bill. Every time I meet him I mostly listen. My intellect isn't even close to his. Zuckerberg, though, for some reason, feels a bit more approachable, but I attribute that to his age and the fact that he's building something I understand a bit more than I understood the internals of Windows.
Zuckerberg, too, is damn smart and in recent conversations with him I've come away thinking "he just is doing better thinking on social software than anyone else." Plus, in addition to his other academic qualifications, he studied Chinese for his recent trip. I was only able to learn a few words. I'm sure he did a lot better! 🙂 I don't remember Gates learning other languages (update: Gary Stein says that he and his wife are learning Chinese), although I'm sure he'd count basic and C++ as other languages.
On the other hand, Zuckerberg seems more comfortable in social settings, walking me right up to Jet Li and introducing us at a Time party. I met Gates at one party in the mid-90s and just had a tough time getting him to talk until we started talking about just-in-time compilers and then he went on for half an hour. At last year's TED in the hallways Bill did the same thing, but instead of talking compilers was talking about nuclear power.
Zuckerberg has impressed me similarly, but is still focused on building Facebook so isn't spending much time thinking about other things. Clearly, as the questioner showed, Zuckerberg is a smart dude too and picks things up VERY fast. I witnessed this when Zuckerberg met other CEOs and discussed the technologies they were using to build their companies.
So, to answer the question: Gates wins, but mostly because we've seen how he learns new topics for a longer time.
What would be interesting is to put the 26-year-old Gates next to the 26-year-old Zuckerberg and compare them. If we did that then Zuckerberg probably would win.