Well, let us put it straight! GitHub uses MySQL. In fact, these guys maybe helped install the new GitHub at Rackspace's data center, but their choices are irrelevant to GitHub choice of platform (the title is bit misleading though).

AS for the subject itself, I guess, if you are more comfort with either one of them, this one would be the best choice for you.

If you never used anyone of them and wish to start a new project which requires an RDBMS, you may search for other comparisons at Google (as well as look at here: http://news.ycombinator.com/item?id=184328)

Most important, no matter what language you are using, make sure to use a database abstraction library, so to make it easy to switch between them in case you are not satisfied with your first choice.