The original was just better done than the later. Better story, better plot progression and better characters. The only thing that Korra had going for her show is better animation but that's simply because it's newer and they used the tech and funds from the last to improve the later.
Both shows were great but The Last Airbender was better, I can still watch it and enjoy any given episode. Korra was more shaky, I hated the focus they put on the poorly handled romances and the show felt less focused up until the later seasons.
This is a tough question! Im not sure. I guess I'll go with Korra. I like her more as an Avatar. Aang doesn't convince me. And I believe Watu is one of the best villains ever. What I like more in the last Airbender is the overall storyline about the war against the Fire Nation and the design. The design in Korra looks too modern which I didn't like so much. Both series are awesome!