The above statement about it having GS on the back is incorrect. The only way to tell is the model #. If it is A1303 or A1325 then it is GS. it should be on the back in very small print, but if someone was trying to scam you they could put a G into a GS case. best way to determine is by going to: Settings; General; About; Model. The model # will confirm.
The difference between 3Gs and 3G is that 3Gs has a faster processor than 3G. 3Gs can attain 7.2mbps on HSDPA while 3G can only achieve 3.6mbps. 3Gs has voice control and magnetic compass while 3G has neither of the two.