There is no difference other than the S prefix 28's are earlier models then N prefix 28's.
Some folks prefer the earlier guns because of the belief that older is better.
Layton pretty well summed it up. I want to ad that all 3 screw guns will be N prefix while S prefix would be 5 or 4 screw and perhaps a few late 60s 3 screw guns. The change from 4 screw to 3 also marked the buyout of S&W by Bangor Punta which many consider the beginning of a drop off in quality of the S&W line. S prefix guns are more in demand and more valuable regardless of if there is a quality difference. My 28-2 is a 1970s N prefix gun and I am pleased with the quality.