A tough question - and one that a club of Uniteds stature should not be considering.
However, if I really had to choose my gut says keep Rashford. A local lad, has great potential, keeps proving people wrong, and most importantly seems humble and hard-working. Do I think Martial is a top player with world class potential - yes. But unfortunately I have come to doubt his attitude, mental strength, and motivation.
I hope Martial proves me wrong and responds to the challenge set by the boss. A truly world class player needs the mental toughness alongside the natural talent imo. Modern greats like Cristiano and Messi have both had to deal with adversity. Ronaldo at one time was a 'one trick pony' and Messi had many well-documented physical issues during his early career. I'm not saying Martial is at their level, perhaps in the future who knows. But one thing is certain, a world class player would respond to the challenge by working hard, improving, and then showing this consistently on the pitch.
Hopefully keep both - and along with Pogba they go on to dominate things for a long time