Sane plays much higher up the pitch than Mahrez in the attacking sense, who likes to drive inside with the ball and pass into midfield (don't get me started).
However, Sane's game is much more prone to turnover of the ball because he plays high percentage football, which we all love him for but the risk/rewards are greater when it comes to quality teams transitioning against us on the break, those are the margins.
Mahrez also played well against Spurs earlier this season and was trusted to cover his full-back at Anfield.
I am Sane's biggest fan but Christ on a bike, he's a fucker still for not getting back in when countered.
Add to the fact Mahrez delivers a whipped cross from far deeper than Sane generally would, it is easy to see why, with Bernado's industry also missing, Pep decided to start Mahrez.
Incidentally, I am not a fan of Mahrez but did suggest in the pre-match thread that it wouldn't surprise me if he started.