Me thinks the Metallica version is better although they KINDA ripped off Dave. They tweaked it. This, in turn, enraged him enough to make the epic album PEACE SELLS, BUT WHO'S BUYING?..right.. Hauler?
Well, that song existed before Metallica. If you look at the liner notes in
Kill Em All, Mustain gets credit for the The Four Horsemen (along with Hetfield and Ulrich). He also got credit for Jump in the Fire, Phantom Lord and Metal Militia.
Mechanix actually showed up as a Metallica song in
Metal Up Your Ass and
No Life 'Til Leather (demo).
So it was first. After they kicked Dave out, Hetfield re-wrote the lyrics and slowed down the riff just a little bit.
I agree - I think 4 Horsemen is a better song. Meatier. More polished.
Mechanix is straight thrash.
There was always plenty of animosity with Megadeth/Metallica. Not sure if Peace Sells was a result of that - but it's a killer album.