Different type of players. One is a passer another a goal scorer. Mkhitaryan has better vision and is more of a team man while Sanchez can take on a defender and beat him for fun. Mkhitaryan can be trusted to play a role while Sanchez excels in having a free role. How do you compare?
Personally we need a player like Mkhitaryan more as he contributes more to build up and can blend in. Sanchez is a very good player who is more of a goal scorer but he can be petulant and get frustrated quickly. We have Auba and Lacazette so we needed a player like Mkhi who can along with Ozil help in creating chances.