Both of these cameras are workhorses, each with their own character. It's hard to choose between one of them. Whenever I have to choose I base my choice on the amount of weight I can take with me. The F4s being 1.28kg and the FM2n being 540gr. you probably understand when I pick which.
Whenever you ask why a photographer uses a certain brand you hardly get a straight answer. I don't know which brand is better, it's just a matter of ‘tradition’. When I was young I first used my parents camera, a Minolta. When it broke a bought a new one because I could still use the lenses so I choose a Minolta Dynax 8000i. (1990)
In 1993 I bought a Nikon F801, probably because it was second hand and cheap. This camera was my companion on all my trips around Europe and Asia. During the digital era it got replaced by a digital brother, the D80 and D90.
But then came the creative urge to shoot film again, dug up my old analogue F801 and bought some film and fell in love with the medium and the art of making analogue photography all over again. In hunger for better equipment, as it was cheaper to buy the professional stuff from when I was younger, I scour the internet and found my former unreachable desire in Japan.
That's why I use Nikon, because I had some lenses and was in love with professional equipment that I couldn't afford when I was younger. Not because Nikon is the best, but because shooting film is the best.