Since Sanchez was in last year of his contract and would go for free - and even if he went to other club in winter for cash, Arsenal would at most get €30mil - I think Arsenal has better deal all round.
You wouldn't get either Malcom or Lemar for €30mil, while Mkhitaryan is definitely player with huge potential and one bad spell can't ruin it. Arsenal has loads of cash (obvious problem is spending though) and considering signing another big name would maybe make Ozil stay.
United got short-term option at most and by 2-3 years Martial will be better than Sanchez (and more useful). Sanchez didn't exactly show great mentality either, and his selfishness was really annoying even as neutral fan.
Who do you think has come of best?
I think Arsenal. I really like Mkhitaryan and i think they will get a lot more value out of him than Man U will out of Sanchez. If Arsenal get a new manager in the next couple of seasons and are pushing for a title with a new attitude around the club, i think Mkhitaryan could be the player that they really need. Especially with the fact that Ozil will be leaving in the Summer and Mkhitaryan will offer the assists that Ozil used to give and the goals Sanchez would give. I cant forget 2015/16 either when he got 23 goals and 26 assists in all comps - shows what he can give.
In the short term, potentially Sanchez.. although i dont know how well he will fit into the team.