Not to pile on with the other replies, but I just wanted to point out yet another difference in the investment models. The BH approach, as I understand it, can be summarized as "Find a good thing and get a lot of it". They find what they consider to be sure things and make really big bets.

YC is just about the opposite. "Find extremely risky things that have huge upside, and invest a very small amount". Both models are great and have proven to be quite profitable. Though to me, YC seems to be much more altruistic. At least currently, they appear to be much more interested in helping people to start successful companies than they are making themselves rich(er). I don't know exactly what pg's motivations for starting yc were, but it looks like it was something along the lines of "Starting a company was a risky decision when I did it , and I want to help others to have an easier time than I did" as opposed to "Gimme gimme gimme money". As a result YCombinator and Berkshire Hathaway probably won't be in direct competition for quite some time, as they are optimizing for different results.