There's not a lot between Martial and Willian at their respective bests. Or, indeed, even during this last season. But when it comes down to it, I'd rather have a player who wants to play for the manager than one who's just rejected a contract to stay at the club.
If Martial is not commited enough to improve his workrate, he won't be commited enough to keep himself fit for the next decade. He may be 7 years younger in real terms, but if history is anything to go by, it won't work out that way in terms of longevity. The players who last the longest in the game tend to also be the ones who work hardest on the pitch. A 7 year difference on their birth certificates would translate into less than that physiologically speaking.
Willian's a sure thing for at least 3 years under a manager not called Conte. Martial's gonna need a couple of years to get that kind of level consistently (if at all) and then will likely drop off quickly if his general lack of workrate is any indication.
If you were to bet your mortgage on it, I think the safer money is on Willian. There's a certain romance to Martial which I totally understand. But it's not a gamble United can afford to take right now.
Also, for what it's worth, LvG was even more defensive than Mourinho. People blaming Jose's style are missing the point. Martial was first choice but then lost his place for footballing reasons. He can't be upset with the team's philosophy seeing as it's actually more adventurous than it used to be when he was happy. He's upset because he's lost the manager's faith, but that's got to be on him. If he'd been good enough, it never would have happened. It's not like Mourinho has been afraid to bench his signings (Mkhitaryan, Lindelof) or big names (Rooney, Pogba) in the past. The position is up for grabs and Martial hasn't been up to scratch. If he wants to give up, he'd get no sympathy from me. 'Bottling it' is the sign of Tottenham or Arsenal player, not a Manchester United one.