I think that comparison is a bit flawed. Sancho's Dortmund is a much better team than Dembele's Dortmund, it's easier for him to gobble up goals and assists. In general comparing the two is a bit like apples and oranges. Dembele was a lot of hit and miss, but his hits were completely outrageous, Sancho keeps things simple, but as a result is much more consistent in his individual plays.
I think if you had to choose you would want Dembele when your team is struggling to dominate/create, like those games vs Bayern, Sancho gets a bit of stick for. Whereas if your team found found its rhythm you'd rather have Sancho.
On one hand Dembele would arguably be a much better fit than Sancho for United, since he can cope better with a RB that doesn't support him and he is much better at carving out something on his own, without the help of well organized positional play. But on the other hand the team is already playing with Pogba, Bruno and Matic in CM and the CBs aren't that rapid either, so if you throw in a winger who gives away the ball a lot and isn't interested in winning it back and you don't have a top level (gegen-)pressing system in place either, teams might just overrun you on the counter all the time.