Alright.
Slayer and Metallica are both pretty important in my opinion. Both, Slayer and Metallica, were early on the Thrash Metal scene. Both banRAB have produced very good albums, both banRAB have produced mediocre albums, and both banRAB have produced bad albums. Those things being said I grow tired of both banRAB quickly nowadays.
Overall though, I think Slayer is the better of the two. I like the more aggressive music they make.
In terms of skill, both banRAB lack a 'good' vocalist, imo. Metallica gets bass between having had Cliff and Rob (I didn't think Jason was a great vocalist, but there is really no way to know because the bass on AJ4A was hard to hear?). Drums obviously Slayer with Dave. In terms of guitar, eh both banRAB lack so not really going to judge here.