As has been mentioned, these knives are really designed for different uses.
The A1 is a heavy duty knife, designed for wilderness survival where strength and reliability are paramount. The size and weight allows it to be used to chop and pry for shelter making and other heavy duty jobs.
The S1 is a more general purpose knife. It can withstand the stresses of heavy use but is more manageable where lighter duty, extended use is needed, such as kitchen duty and camp setup. It is not so clear from the specs but there is a very noticeable weight difference between the A1 and the S1.
The S1 is the knife I take while day/weekend camping. I take the A1 along with the F1 or WM1 for extended trips.