I brought sales into this because of the recent argument that Slayer is a bigger band than Megadeth which is not true in the least.
And obviously there is a bias on all fan bases. A lot of people will say that Cannibal Corpse sounds the same on every album, and me being a fan I will say that isn't true. So there you go Slayer fans.
In terms of musical quality and ability, Megadeth. In terms of overall thrash, Slayer. In terms of overall success, Megadeth. In terms of starting more extreme genres, Slayer. But overall impact on metal, Megadeth.