Given a choice of FM2 versus FM, I would go with the FM2. Better features and newer design. Of course, you need to take into account the actual condition of both.
I've had an FM2 since 1982; it's still my main 35mm SLR. Excellent camera, very rugged. I put my thumb through the shutter curtain shortly after buying it (now that was stupid), and about 1992 I sent it back to Nikon to have the entire shutter assembly replaced. It's hadsomething like 1000 rolls of film through it - very rugged machine.