I mainly do Jungle and Mid/Top, Im at 5.5k IP and I can't decide who to choose, I watched videos of both of them and i really love them both but i can't just decide!
Diffuculty doesnt matter much, just tell me who is the better pick?
Now 
to me, Lee Sin brings more to a teamcomp and is more versatile. You're also less likely to be called salty.
I like lee sin more, as when you kill an enemy, you knwo you outplayed him. To me, yasuo is one of, of not the, most unfun to play vs champ
For Yasuo, most of them I've seen NEVER use their ult because they can't time it right. If you can though, you'd be pretty good I guess. Yasuo has some advantages over certain champions, but when he doesn't, he fails. Hard.
For Lee Sin, he's really strong early game but gets weaker late game. He's good for jungling and someones top/middle. I think Yasuo would require more practice than lee.
Lee sin is overpowered, buy him.
