I believe football has come into a new era. The amount of money at stake is much higher than even 10 years ago. The biggest clubs make far more than 2nd tier clubs. Utd make far more than most of the big six, and the pressure to win now is immediate. This is the same problem than Bayern, Juve, Real, Barca and PSG face. As such, I believe that clubs cannot wait for players to develop, they need to buy proven players that can perform on the big stage.
I rate Martial, he can be fantastic on his day, but he needs to improve immensely to play in a World Cup final, or a CL final. Just not consistent enough to be on the team sheet every match. Lingard, on the other hand, is not as skilled, as dynamic, and cannot provide the moments of brilliance like Martial, but, his work rate is insanely high and his movement is fantastic. The ceiling is higher for Martial, but at these high stakes, it may be too much of a risk to see if he can improve his consistency.