Sancho strikes me a much more a Liverpool type player, powerful runner in straight lines and a great engine but not much finesse to his game at present. Something I've seen on here and agree with is that Sterling is better on the right side while still being effective from the left. It's worked really well to have Sane as the left choice, Bernardo/Mahrez for right and Sterling a great option for both sides to keep opposition guessing. I've only ever seen Sancho playing on the right so signing him would make Sterling our full time left winger and waste a lot of his versatility.
It's a real shame if Sane does want to leave as I think he's pretty unique compared to what we could get to replace him. There are any number of decent right footed inside forwards for the left but I don't see many who provide what Sane does, certainly not at the same quality.