The Legend of Korra had a better setup to me (the bender vs non bender conflict). The villains could have been quite compelling and well developed. Also the romance between Korra and Mako was better for me than the romances in ATLA. Unfortunately, Korra did not live up to its potential in my eyes and therefore, I don't think its even close to ATLA.
1) The plot is rushed and there is not enough time to properly develop characters. Amon's backstory is squeezed into an expository chunk in the finale (you'd think a guard would have come to check on Tarlok in that time).
2) Miracles happen in the finale: Korra conveniently learns to airbend when Mako needs her to (even though we've seen no progress since episode 3). Aang's spirit somehow manages to give Korra bending back and instantly teaches her how to restore bending to others.
3) Its never explained how Amon could convert blood bending into taking bending away. If Tarlok could explain Amon's backstory, surely Amon could explain how he managed that.
4) Both Tarlok and Amon seem to repent for their sins (Tarlok says he's sorry) and Amon tells Tarlok he wants to start over with him like the 'good old days'. Are we supposed to feel sorry for them when they die?
5) Pro Bending comes off as a complete distraction from the series true plot. It either should have been eliminated or (if the series were actually given the time it needed), should have been the focus of the first season (I think the episode where Amon crashes the pro bending championship would have made a better season 1 finale).
As for ATLA, its far from perfect. However, it has three 20 episode seasons to tell its story. Korra only had 12 episodes. Zuko's backstory is revealed over the course of a few episodes instead explained in one chunk as an attempt to make us feel sorry for him. However, the villain (firelord Ozai) has a less compelling cause than Amon (at least until we found out he was just using the equalists to gain power).
The series finale was also a disappointment for me, but that's because to me the series had set my expectations too high (ironically I also thought that this finale should have been longer by at least two episodes to develop its plot more). There was also the little deus ex machina at the end that allowed Aang to escape his moral dilema, thus rendering the wisdom the Avatars had given him nigh useless.
I also believed that if the were going for Katara/Aang, then Katara should have been with Aang when he fought the firelord and not with Zuko fighting Azula. The series did not do a good job making the firebenders seem all powerful with the comet's influence (only the guys who were supposed to kick butt did so, the others just seemed normal). I could go on, but basically, the bulk of my complaints about ATLA are with the series finale, while the complaints with Korra are for the series as a whole.
For all its imperfections, ATLA does a much better job using what its has set up to tell its story. It has more time to tell its story (thought Korra will have a second season that will hopefully redeem the disappointing first). Its (main) characters are more complex by the end, while Korra's characters seem flat and dull.
Watched both series fully.