Previous experience is the key, regarding both the mouse flying part and the flight sim part.
You take a person fresh to both the mouse and flight sims I'd say 99.9% of the time a person who uses a joystick over a mouse will have it better.
IMO, it's better to buy a joystick right off the bat and fly off-line or H2H free and get used to the flight model, gunnery, etc. Once you have that start paying and you'll be a step ahead in the MA, CT or any arena.