The version I is the original older 50mm f1.8 with metal mounts. Its built is excellent. The version II is the same lens in plastic mounts. Its cheaper because of its new plastic parts. Optically similar with some users prefering the older version I.
Back to the zoom-prime topic. I love zooms, but there are times when I take just the 50mm or 28mm out for a spin with the FM2. Very small package, gets the job done.