Also there's the curved type of 'rain sensor' mirror, and the squarer 'rain sensor' mirror. Both FL, but the latter was later. Appears both were fitted to Vectra Cs also. The earlier curved mirror looks identical to the standard Omega mirror, but as said, different mounting bracket.
Easiest way to tell, there's a captive curvy/oval beige trim panel on all standard mirrors, all the 'rain sensor' mirrors have a rectangular black trim panel (which hides the rainsensor).. or this is missing. In which case you still can't confuse it with the standard mirror because, of course, there's no beige trim panel.
Elite mirrors all same... except later cars have a different plug. Not a problem if you have the plug tail to go with the new mirror. Equally not a problem if you don't, just means getting the soldering iron out
Wire colours change too, but only the traces iirc... Haynes is accurate enough for this job
DBGs info above is slightly misleading...
Elites all have mirror memory, so contain a couple of motors... hence the bulbous shape and weight.
Lesser models, except GLS had an optional Pack, (Visibility?), which for £200 ish got you auto dimming mirror and rain sensor. This mirror contains no motors and is much lighter/flatter. The visible difference being the light sensor at the base of the mirror. It is this mirror which is the same as that fitted to Vectra C/Signum Design and Elite models.
Equally important... all Omegas share a common mounting, so mirror will fit. Only difference is the screen as the shading is different on rain sensor equipped screens.
Hth