When it comes to balancing champions

both at the same time if a champ has 55% win rate but only played by experienced people because his pick rate is 5% then he shouldn't be nerfed

that's why fiddle and rammus had a broken win rate for 2 or 3 years

depends on your balance goals. Winrate is more of an issue at higher levels of play. Popularity can also be an issue if it's completely skewed to the point where there aren't enough viable options and games are more or less won at the pick/ban phase. I don't think this happens that often with exception to when big patches that buff multiple champs a lot happen.

I would say for compeititve it's more about winrate for casual players it's more about play rate.