To me, with that part subtracted, both versions of the same. Goofy lyrics about the apocalypse weren't any better than lyrics about bangin a chick. To Metallica's credit, they managed to twist the riff and make it give off the feel of galloping horses whereas Mustaine executed the song to that of a whirrng car motor. It speaks volumes to the versatility of the riff, but that middle section is what makes it undebatable that Four Horsemen is the better song.
All kinds o fuckin dinosaurs!