Dave Mustaine was kicked from Metallica after his problems with drugs and alcohol and becoming violent. Though he held a short grudge, he set a goal that he would become better, faster, and heavier than Metallica. Therefore, it is for a fact, that Megadeth is the better, more consistent band. Metallica is still a good band, nonetheless. Dave Mustaine was the lead guitarist of Metallica, and thus is skilled in that area as well, and has a place in playing about 40% of Megadeth's solos. Although his solos aren't excellently clean, they are THRASH! His lyrics are also very well-written, and meaningful.
Those who bash Megadeth, saying things like "fuk off megadth iz the worst band in the wurld fuck you and all of you megadeth bitchs, go metallica!" ... Clearly, you're uneducated, the only words that are projected from your mouths are 'Metallica' and 'fuck', and have not heard more than three Megadeth songs, or enough to get used to their style.
Mustaine wrote songs like Ride the Lightning and The Four Horsemen (Mechanix), which his style can clearly be heard in. As Metallica progressed after Mustaine was fired, musically they strayed from Mustaine's style, allowing for a decline in the quality of their music.