Kindred or Lee Sin: Who's harder to master?

I see a ton of posts on here along the lines of "who should I main" and invariably when either of these two come up there's a number of replies that say "oh no those are really hard just run Noc/Hec/WW/etc"

Well sure. Yes. They're harder to master than say Amumu. But if one were to commit to them, which is the harder of the two to learn? Which, do you think, is more rewarding to put the time into to learn?