If Star Wars is taken to mean just the original three films, and Star Trek just the original series and the first six movies, Star Trek wins, but by a fairly slim margin.
If we're talking about the whole thing, all the series, the prequels, the cartoons, the omnipresent, all-encompassing torrent of shit that compose both franchises, then they've both mediocred themselves out of the running, and Firefly wins.
That's right, I just made the substantive mediocre into a verb. Fear my power.