Slayer/Megadeth

Well, I did say "arguably". I think it comes down to technique. Lombardo is fast, but he's also sloppy, and that's ok for what Slayer does. Metallica needs stricter timekeeping because the rhythms and changes are so tight; they're an interesting band because it's not really Lars who keeps time. It's Hetfield.
 
Yeah, I have heard some interesting things about Lars playing along to James and not the other way around. Apparently, Lars only has James in his feedback so he can keep closely in time with him. This is one theory as to why the mix on ...AJFA is so different. Lars got behind the mixing desk and made it sound like what he hears, ie, all drums and James, and then solos put over the top. No bass.