In an interview with Guitar World there's no competition between Megadeth and Metallica. Mustaine said Metallica got a "big head start" thanks to him and the music he wrote for them.
Mustaine said "In my mind, there is no competition between Megadeth and Metallica. We're different bands, and I believe Megadeth has been more consistent. But the sad thing is that the drama between us has been more popular than the music ever was. And remember, Metallica got a big head start, and they did so on the back of what I helped create."
Talking about Metallica Mustaine added "They became one of the biggest bands in the world, and here's one of the biggest bands wasting their breath trying to discredit me by saying, 'Dave's not a good guitar player.' Excuse me, what the fuck did you say? [Laughs] I think I wrote many of the songs that made you famous, so you probably should recheck that bullshit statement. But this is the shit those guys say, and you've got the sheep who follow them around believing it."
Megadeth Merchandise
He also talked about Metallica's longevity and success over Megadeth saying "A lot of it is the take-no-shit attitude. People know Metallica has always tried to hold me back. They never expected me to do what I did after they fucked me over, but I succeeded and made better records along the way. But I don't care because I'm happy with my success, and I've had plenty of it. The music we make now tells you all that you need to know. But I hope we can tour together before it's all said and done, but I don't know if we will.
"As far as looking back at everything from the beginning, I think I've maintained my integrity. When I injured my arm, I sold off all my gear to pay the people I owed money to. When most bands end, they say, 'Fuck it,' and leave their vendors holding the bag. Not me. I'm way different. I'm old-fashioned. I'm old-school. I do what I say I'm going to do. That's why Megadeth is still here."
In conclusion, there are still a few grudges Mustaine holds and there's not much water under the bridge. Maybe one day, in one world Mustaine will join Metallica on their next album. What would that be like I wonder.