Personally I think it's wrong to say one guitarist is better than the other. Styles and personalities differ and that may fit a band better or worse.
Fair comment, I've never agreed with the idea of one musician being "better" than another, it's not athletics.... although I have probably slipped into saying it more than few times.
In this case I'll just say that,
IMO, Megadeth made better (agh, that word again!) music with Marty Friedman than they have done without him.
Mustaine's comments there sound defensive and bitter - I don't know how successful Megadeth are now, but I very much doubt they've "achieved more" (commercially or artistically) with the current lineup. And were they "already huge" when Friedman and Menza joined the band? Maybe..... but they'd just made their worst album, with their worst lineup (
IMO!), and seemed to be in self-destruct mode. And "huge" or not, they definitely got "huger".
I've always been surprised how complimentary Dave was to Marty in the years after he left - I think he was hoping he'd come back. Looks like the resentment has been building up, and now it's spilling over.....