The alloy frames were cool, but(based on everything I have read) were not long term durable.
Allegedly the MkIII cast frame came about when they did the .40 S&W and carried it over to 9mm due to being more durable than their forged frames. Forged frames are smooth machined on the bottom of the heel(behind the mag well) while the cast frames have a textured appearance.
Another thumbs up to the MkIII being set on sights, safety, hammer profile that won't nip most hands, and pretty much every bullet shape friendly feed ramp. The mag disconnect is on almost all of them, but is simple to remove.
From what I recall, MkIII is no firing pin safety while MkIIIS has it in place. Not sure of any external cues to look for on determining which it is.