He is also way more proven as Martial. I won´t comment on his potential, because I have only seen him a handful of times in the CL and the only thing about him that stood out to me was his first touch and close control. Maybe he is this mega talent and the transfer will pay off quickly, but it is also dangerous business if he is the only one coming in today. You pay this huge chunk of money not for immediate perfomances but for potential.
He basically replaces Januzaj (at least for this year) and it would be certainly a sign of Van Gaals confidence if he thinks it would be enough to trust a still fairly raw talent to help you in the area you arguably struggle the most, which is creation of quality chances, and secure a top 4 spot and a good campaign in the CL.
The thing I worry about is, if you guys have really the luxury to develop a talent and at the same time introduce him to the EPL. And with development I also mean the negative things that come along with it: likely inconsistent performances, freedom to make mistakes and still be played on a frequent base. Even Eden Hazard, who set the French league on fire in his last season there, faded for a good portion of his premier season after his strong start. If Van Gaal sees United in his current state as the right spot to mould him then fair enough, but I could also see a 19 year old struggling with the pressure and expectation this price tag could create.