If you really want to compare them seriously, I would say that Sané has a more limited game but he's better at doing what he does.
Sancho's got a lot more variety because of his two-footedness, he will make passes you'd expect from a number 10 that Sané wouldn't ever attempt and he has different types of finish as well.
Sané has raw speed that's undefendable when he's in space, but we've seen his play time limited because if you do take that space away by parking the bus, he can become a bit of a spectator. Sancho doesn't have that speed, but with no space, his dribbling ties people in knots, similar to Bernardo at his best in the box. Sancho's final ball on counter attacks and the like seems better as well, that's a part of Sané's game Pep always used to mention needing improving.
If you compare Sancho to Sané and Sterling at the same age, he's a long way ahead, and that's not surprising because he's been ahead as a 15 year old, 17 year old etc. but he needs to play under a top coach next to make the most of all that potential. If he goes to Pep or Klopp, maybe even Pochettino or Nagelsmann if they end up at clubs who can afford him, then he'll be that generational player. If he goes to a manager who doesn't focus on improving their players then he'll be very good but not great.