On the whole I prefer Slayer but if you put all of their albums in front of me, my favourite would be the Black album which is of course Metallica. Slayer are getting better and better where as I think Metallica have had their hayday.
(pst.. Soil are dominating my metal morning at the moment)
Early Metallica takes some beating. Sanitarium and One being particular favourites. I never quite 'got' Slayer though. I appreciate they're more musically talented when put up against the likes of Deicide and Prong, but feel they still lag behind the likes of Metallica and Pantera. Don't get me wrong, I love South of Heaven, but I feel Metallica win this battle simply based on their early stuff. Their latest stuff sucks though.
i personally would pick Metallica cause im not into the whole "satanic" thing and the death metal screams but thats why its hard to compare them. just accept they are two extremely talented bands.