They are both fantastic. Right now I still like TLA more but Korra is an ongoing series. Maybe once it eventually ends I'll be able to fully decide, based on TLOK's ending vs. TLA.
The Last Airbender solely because all 4 books were planned to be a story, Korra started with just season 1 everything after is after thought. Korra is still good and I really liked both seasons but there's just little things that make me like it less than the original.
Avatar the last air bender, ONLY BECAUSE, it lasted longer and I grew more connected.
However I love Korra, and I need to catch up I just forgot what episode I left on at :'/
I like TLA better because the plot just flowed so smoothly, and I like aang a bit more. Also the whole underdog thing was cool, and I like the more primitive culture better.
I like TLK better because it has some really interesting things going on. It goes deeper and further than TLA. It does things like go all the way back to the first avatar. It might be better, if it just flowed a little better.
legend of korra is great but TLA is better. Aang and 1st team Avatar is just more appealing and better character than the ones on legend of korra. also better plot and action