General summarizations of each:
Atla: childish on most matters, a lot of things were mysteries at the end. Did a great job with 4 teens travelling the world and introduced great aspects. Aang would be easier for children of most ages to connect to his really nice and fun and silly personality, he's a boat load of immature through most of the series.
Lok: touched more mature subjects, things were explained a lot better. Had a majorly better variety of everything and further developed bending. I think it was easier for people in general to connect to korra because of her personality and because she's closer to our time.