I only own the first edition so I do not have a comparison. One would typically think the latest version would be the most complete and up to date with any errors or admissions being corrected. I would also think (and hope) the latest version would have improved pictures and illustrations.
I don't own any S&W's manufactured after 1994 and therefore I am not in the market for a new edition. The original version taught me what I needed to understand to repair anything I own. If you gravitate towards the vintage guns, than I suppose the least expensive one will do. If you see yourself or actually own newer models, then I'd go for the latest edition and again, if the illustrations are much improved, the latest version is probably the way to go as well