I now had time to do some macros for both watches.
To make it quick: the milky ring of the sapphire crystal is distinctly thick on the FOiS.
The color of the FOiS dial is significantly darker towards black and looks a bit shiny. the surface is a little rougher and the print a little thicker. the radial rings of the subdials are less prominent than the ones of the 321. the bevels of the case are sharper on the FOIS, and on the 321 they are smoothly ground like a baby's bottom. The finish of the caseband is compared to the 321 a bit less fine. The ceramic bezel of the 321 has a different „printing“ quality but the aluminium bezel looks also very nice.
I do not have the opportunity to meassure the width between the lugs but it seems as the lugs of the FOiS are a little bit wider.
I don‘t want to change the bracelet of the 321 to the FOiS because I am afraid of scratches so I have to apologize the poor pictures to compare the lugs.
Overall I like both watches very much and they have found a cozy place in my Omega watchbox. have a little rum watching the pictures.