Megadeth is better IMHO. There's no doubt Metallica are more media-friendly and get more airplay. Most of their songs pre-Load are pretty good. Their last three studio albums have gotten progressively more spotty. St. Anger is barely listenable. I have high hopes for the new album though I don't know if I'm just kidding myself.
On the other hand, Megadeth put out great metal albums until they hit Risk as a low-point. Their three studio albums since then have gotten progressively more amazing. United Abominations not only destroys Metallica's stuff since MoP, but (again, IMHO) also destroys most of the music being put out so far in this century. Top it off with Mustaine organizing Gigantour with bands like Lamb of God and Anthrax and you have one guy very devoted to metal.
Megadeth all the way!
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