Half an inch of barrel is not going to make any difference in accuracy.
I prefer S&W adjustable sights, not because I can move them, but because I can SEE them better than fixed.
However, the 3" gun has the advantage of a full length ejector rod; the short rod on a 2.5" revolver takes a good whack to clear the empties, and may not always work then.
If you already have both a 2.5" and a 3", try both and see which you can shoot (and reload) better.
If you have one or the other, shoot it before spending more money.
If you have neither and want to break into SSR, forget the snubbys, get a 4" adjustable (M66 in K frame) and don't worry about the details.