Now that is a question oft discussed by teens, I would like to dwell on that matter for a while solely for the reason that these were my favorite bands while I was growing up.
It is a bit tricky because Megadeth IS Dave Mustaine however mustaine was a part of Metallica too, so you don't know whether the earlier Metallica stuff is actually Megadeth or Metallica.
I shall leave aside these things and focus only on my totally biased opinion.
As far as selling out is concerned, Metallica are guiltier than megadeth. One major reason being: Black
Megadeth not being far behind with cryptic writing and the infamous Risk.
Virtuosity being an even-stevens between the band with Hetfield and Ulrich on one side, and Marty Friedman and Mustaine on the other, though I still give the edge to Megadeth. Also I have got to say this, Hammet is not very talented at all, I resisted from accepting this fact back then, but now I see.
Musically, I'd guess that Metallica is a bit more melodic but I guess that is a direct consequence of their 'selling out' but megadeth has sheer power, which I love.
If you look at who has more balls I'd definitely vote for Mustaine, a bad ass mother fucker, no compromises whatsoever, he kicks Metallica's ass just with his integrity.
And all this from a Metallica fan
(Here is a secret, I can't listen to Metallica anymore :( sad but true.)
Listening to Tender Surrender - Steve Vai
"Hark! yonder imbecile, amuse me.Now." - SiegerKranz Meer