I don't think the 'better league' is of much value. Both teams are in the same class of teams. Even if City might have been better this year.Playing in a better team and a better league against better opposition. He was a regular starter last season and for significant parts of this season. Pep has shown he changes his mind on people if they put in the hard yards (Aguero, Bernardo, Zinchenko, Gundogan) so every chance he can become a consistent starter again. Particularly if he shows he can play as well on the right as he does on the left
And if I'm being honest, I don't see a path to him being a consistent starter ahead of Sterling or Bernado. Both seem to be growing in clout and accomplishment at raters faster than Sane.
On the other hand Bayern just lost 2 legends at Winger and have none half as good as Sane. Here he has to compete with 2 who are clearly better a d a 3rd that's currently as good.
I know it sucks, but if I was Sane I'd just ask Bayern to match whatever City was offering and I'll be on the way over.