Brains, like beauty, are in the eye of the beholder.
While some people believe you can measure intelligence (hence, the I.Q. test); there are others who believe that it is not what you have, but what you do with it, that matters.
Just as an opinion of which is the smarter of the two gentlemen you name in your question ? I'd say Albert Einstein. Bill Gates may be smart enough, but it seems to me his smarts come from knowing how to build the proverbial 'better mousetrap.'
Einstein could have taught the mouse how to evade it.