For scouts,
MGE = Best for practicing aim because the opponent will always be focused on making it hard for you to hit them, while in DM you spend a lot of time shooting soldiers running in a straight line in the back. Obviously, Its really good for practicing for 1v1 situations. I find scout vs soldier MGE to be really good practice because you get to work on your Projectile and Hitscan movement.
DM = Best for practicing group fights, Playing in disadvantageous situations (2v3, against a soldier while youre inside of the house on badlands etc, hitting the easy shots on unaware people), and proper movement when more than 1 person is firing at you.