I think some are getting carried away. Mahrez slots perfectly into the RW role at City, much better player than Sterling or Bernardo. He was more productive last season than any attacking midfielder at Utd. Additionally, Mahrez has his prime years still ahead of him. Sanchez has already started to decline, especially his decline in numbers is staggering.
The Sanchez deal was good because it was a swap with Mkhi, who is same age and nearly useless in the PL against any kind of good team.
But if you honestly pose the question who Utd needs more, then the answer would be Mahrez everytime, to fill in at RW, while Sanchez takes playing time from Martial and Rashford. Baffling squad planning.