Tbh, looking beyond goals/assists and potential, Martial doesn't have much on Willian. Martial would even do well to better Willian's level on the long run as he kind of lacks the workrate and attitude needed to be truly worldclass.
His lack of dynamism means he's easily shackled and more often than not, he looks disinterested and struggle to impose himself in games.
If you take a look at the best players, even when they or their team are having a bad game, they still harry and huzzle to get more involved and to avoid anonymosity which is the worst - unlike Martial.
Willian offers far more than goals and assists; he runs all day, he's one of the best at transistions, very good at set pieces, one of the best ball carriers, he hardly loses the ball even under immense pressure and he more often than not, exerts big influence on the game.
He might not be the superstar type, but I can't help but think he's the final missing piece in our attacking jigsaw - unless there's a similar but better player.
Willian is the type of players you need to win immediate titles, Martial is the type you use to try and build teams. But considering the fact we have a very good Sanchez and Lukaku for the positions Martial excel in but a non-existent right side, we would actually be smart to make this deal.
Ranging from speed, movement, workrate, crossing, a right wing threat, transistions and to a lesser extent, width, Willian will offer us most of the skillsets the team needs and solution a few problems.
