Some Sigma lenses are designated "DG". What does this mean?
The DG designation applied to certain newer Sigma lenses (mostly wideangle and wide zoom type lenses) indicates that the lens is especially suited for use with digital SLR cameras. The DG lenses feature improved (more even) light distribution from image center to edge, especially at maximum aperture, compared to conventional fast wideangle lenses. This is important in digital photography, but is also useful in 35mm photography, especially when slide film is used. Also, the shorter focal lengths are desirable, because most digital SLR cameras have image sensors whose dimensions are considerably smaller than a 35mm negative (usually by a factor of 1.3X to 1.7X), making the lens angle of view equivalent to that of a longer lens on a 35mm cameras.
...but I'm not aware of a test between an "old" lens and a "DG" lens. Maybe 50% marketing and 50% improvement.