I think probably they influenced each other to some extent as peers in the California thrash metal scene, but I doubt it was the Clapton/Harrison relationship you're making it out to be. I'll agree that Slayer prob influenced Metallica more than the other way around though. Slayer was never much for the prog stylings of Burton/Hammet, but you can see Metallica attempting to emulate a faster and more frenetic flavor of metal in tracks like Damage Inc. and Dyer's Eve which were a departure from their slower proggish tracks.