The EM, while one of my favourite cameras, is quite limited, being essentially an Aperture-priority only body. If you like the feel of the body, the Nikon FG is essentially the same camera with full manual and a Program mode.
Otherwise a Nikon FM or FM2 would be a good choice as an upgrade. Or an FE or FE2 which will allow you to shoot in Aperture priority like the EM or full manual.
Sticking with Nikon will allow you to use the EM as a backup body, and to keep different film in both bodies. Not to mention the obvious advantage of using your current lenses on the new camera.