Whichever one you practice and play the most
I think he's asking which champs are stronger in general.
Mainly, it depends on your skill level and who you're playing against. For example, if you have a lower summoner level, playing at a low ELO, or just playing against noobs in general, Sion is a pretty solid champ to pick due to many factors. His passive and ult give him solid sustain, his stun gives him great ganking/good at surviving ganks, can work well in solo top or duo lanes, can fill the role of offtank very well, and in general is very beefy thanks to his E. This makes him seem like a freight train, especially late game.
However, once you start playing against more skilled players is when you start to run into trouble. Sion suffers hard early game if he's laning against multiple ranged champs and can't farm. No escape tools (stun isn't long enough) to get away from teamfights, once he starts, he is committed. His builds are usually pretty expensive, so basing and denial of farm hurts a lot. Also, his shield bursts pretty easy.
