The 28X is part of the Omega 30mm family dating back to 1939 or so. It is a great reliable workhorse which was used up until the mid 1960s and replaced by the 60X movement which was I think developed alongside the 55X Auto and is 28mm in diameter. Functionally there isn't much in it, both are robust and reliable. The 601 has I think maybe 4 hours more reserve. Purists would probably go for the SM30 but buy the one you like the look of most. The 286 lacks the Breguet overcoil balance of the earlier 30mm variants by the way. The 285 and earlier are posher in that regard.
Some info:
http://www.ranfft.de/cgi-bin/bidfun-...uswk&Omega_286
http://www.ranfft.de/cgi-bin/bidfun-...uswk&Omega_601