The FM2n is all manual and mechanical, has an LED meter display. The FM3a is sort of two cameras in one. It's both a fully mechanical camera with the full range of shutter speeds if the battery dies, and an automatic camera with electronically controlled shutter speeds. So it's both fully mechanical and fully electronic. It has a meter needle instead of LED's. Both cameras have a 1/4000th top shutter speed and interchange most accessories.
The FM3a is very collectible, because of its unusual design, its status as the last of the FM/FE/FA type cameras, and because it did not sell in large numbers. IMO, it was a Nikon tour de force, displaying their prowess at camera design and building.
If you just want a camera and don't really need or don't want an automatic camera, I say get the FM2n and spend the difference on film. If you want an automatic I say get an FE2 and spend the difference on film. If you want a very special camera pop for the FM3a.
Just my opinion.