The music in Motorhead's version is pretty cool, of course it's more polished and they had more money to spend on a good recording than the original and much rawer original by Metallica. However the moment that Lemmy opens his mouth and starts singing, the song immediately begins to sink for me. If everything Motorhead did was instrumentals or if they had a different (read that as 'better') lead singer, I might feel differently about them. As it is, Lemmy's vocals bug me.
Metallica gets muh vote.