Lo Celso is much better at stopping breaks because he can actually read the play and stop them before they get too far.
Sissoko is so often out of the play that people get impressed on the few times he actually works to get back so it looks like he "puts in work" or is very quick but the reality is more often than not he is trying to recover from a play he should have made earlier.
As for him being strong he has repeatedly looked pretty weak getting beat on high balls this season, highlighted by him doing nothing on the West Ham goal earlier this year.
Lo Celso definitely has the pace to get back, he is a better tackler than Sissoko when he does get back and even better he is so much better at stopping the attack so that he doesn't have to drop back so deep as Sissoko does.