Toph and Zuko denounced Aang's skills at earth and fire bending and the OWL who dislikes Korra almost as much as this board says she mastered those two elements along with water bending. And she can heal which Aang has never even attempted. And please you can whine and use the "f43ts without actually posting f43ts" argument but Korra's feats at everything but air is just as comparable.
Lack of feats is attributed to the producers choosing the cheapest production company which has issues with other shows like Naruto.
Aang as a kid sucks at everything but airbending and crying.
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Honestly this isn't a fair matchup we've so much more with Aang and Korra has yet to show us her ultimate potential, I mean in my opinion she is way better trained than Aang she learned from masters while Aang had to learn his bending from Katara which isnt much of a master(Toph and Zuko are exempt of this for obvious reasons) that said Aang has shown more feats with his bending and his Avatar state. I'd say he gets the win.
Never on screen, but I don't see why he shouldn't be able to. He's got Earth Sense same as Toph and he was around when she opened her Metal Bending school. He easily could have learned at some point despite Su Jin saying Korra was the first.
But considering this is Book 3 Aang, no he hasn't bent metal since Toph just learned.
And since this is Book 2 Korra, she can't metal bend either since she's just now starting in Book 3.