The original series shyts all over Korra. It has a better cast, it's funnier, It was a more cohesive story, it maintains it's consistency and gets better thought. Korra had one great season (3) one solid one (1) one acceptable one (4) and one crap one (2)
Meanwhile the worst season of the original series was the first one because it started a little slow but by the midpoint it was good and it never dipped below that. Korra's quality was all over the place and the second season was so bad it almost killed the show.
I like both series, but as far as the story being more cohesive, that's more on the network than the writers.
The original series was planned to be three seasons from the start, so they planned the story around that. With Korra, it was originally going to just be a miniseries, and they didn't even know if they would get a second season (much less more) so they had to make each season a self-contained story. Otherwise they'd run the risk of having a cliffhanger that never got resolved.