I wasn't a kid when I watched those shows for the first time, so I got nostalgia glasses for neither. I watched The Last Airbender on TV when it was coming out, The Legend of Korra I watched a few years after it was completed, but haven't rewatched them till I started this page. Watching stuff for a second time always gave me a different pespective, there are movies and shows that I came to appreciate way more after a second watch, and others way less. Korra is definetely a show I came to appreciate more.
First is the animation, a total killer. Coming out later there was a lot of technology to help Korra look amazing. Better lighting, particles, and a lot of 3D effects for vehicles, buildings, props and background in general, that helps a lot with depth and dimension, but the overall directing for fighting and action scenes are also on another level. From the whole bending to aerial dogfights and people flying and being thrown while the camera flies through the scene... I can't really compare it to any other western TV animation series from its time or before.
The story and characters are way more mature. And this here is personal preference, but I like how dynamic and unpredictable it is - whereas Aang is an Epic, and in the 30 seconds opening it already tells you all you are going to be watching on its 61 episodes. Honestly, by how rich the story of each season of Korra is, any of them could be stretched into an epic too. The villains are also on another level. I love Azula, but Ozai is pretty unremarkable, they had a lot of time to build his character but he was ignored. Amon, Taarlock and Zaheer, on the other hand, are great! - well, I find Unalaq and Kuvira just ok, but still more interesting than Ozai.
I feel like Korra falls short in synergy between characters. Having way more of them, while an average 13 episodes per season, also doesn't help. Yeah, Mako, Bolin and Asami can't really compete with how great Katara, Sokka and Toph were together, and there is also the issue that the character's in Korra are always drifiting apart, and they didn't get whole episodes for each to develop them further like The Last Airbender had, so the main cast is not as memorable, but I find Korra a way more interesting character than Aang - and, Iroh aside, same for the supporting cast.
Fandoms tend to be just toxic congregations of manchildren sucking hard on the nostalgia cock, proclaiming whatever they watched when they were kids is the best thing ever and everything released after is shit (so many big franchises are on this endless cycle...). I remember back in the day Korra got a lot of hate, it was Avatar but it wasn't Aang so "Booo!", but Korra is a fantastic series, with way more mature and serious characters and better written villains and story in general. The Legend of Korra is way cooler than Avatar: The Last Airbender!