I thnk the reason Slayer headlines over Megadeth is due to th fact Slayer has its most popular line up in tact while many people still want Menza/Friedman to reunite with Mustaine/Ellefson as this was Megadeth's most popular line up. While I slightly prefer Megadeth over Slayer, I can see why Slayer headlines over them at this time. Thats no disrespect to the current Megadeth line up - Chris Broderick is a phenomenal guitarist!!!
1-13 Philip H. Anselmo & The Illegals