hetfield been playin guitar way longer then hammet though, and hetfield is a pretty amazing guitarist,but if anything i'd say he's the heart of metallica, hammet is pretty damn technical and can do some amazing things, but if you had to say why metallica has had the impact it has and why it has pumped out classics that are just bloody amazing (master of puppets is awesome) i'd say hetfield is a pretty major part of the how and why. hammet trained under satriani and started when he was 15 i believe, hetfield played from when he was onli lil.