The two are very different shows, mainly down to the number of episodes.
12 episode Seasons vs 20 episode seasons,
With Korra Book 1 we got such a more action packed and intense story than ATLA book 1, in part because Korra as a whole was planned as the 12 episodes, but mainly because the episodes really just fly along.
ATLA at the moment is better to me, just because we have seen the whole series, and we have only seen Book 1 of Korra.
Only thing Korra is missing is more moments for the characters to just develop as characters.
But Korra was so good in Book 1, and has so much established that I think Book 2 could be the best Avatar we have ever seen.