The Four Horsemen
I'm not sure I can vote because I like both songs. In general I prefer Megadeth and their version has a much better intro. However, Four Horsemen has that cool interlude and better production values. I guess the tie goes to Megadeth since as several before me have pointed out, it's Dave's song. If you're familiar with both Dave's and James's rhythm style, it's pretty easy to listen to Kill 'Em Al (and to a lesser extent Ride the Lightning) and figure out who originally wrote what.
The Mechanix for the music & Horsemen for the lyrics but overall i voted The Mechanix.
I like both songs a lot, but as a song, the Metallica track takes it for me. The pacing, the tempo changes, production, lyrics… no contest.
As far as lyrics go, I can't understand most of them in the Megadeth song. Are half of them in French? I do like the guitar tone, though, and the energy.
The Four Horsemen easily . Maybe if the fist Meagdeth album had a halfway decent production I could have gotten into it way back in '85 but i thought it sounded like a mess