I like both better in different ways.
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.