EX is their designation for their top of the range lenses and IIRC they come with an extra couple of years of warranty. That would suggest to me that they're possibly built to a higher standard than their "bog standard" ranges. The DG mentioned in the thread title is a reference to the fact that the lenses so marked are suitable for use on full frame digital and 35mm film bodies, while DC indicates that the lenses are only suitable for APS-C (or smaller) sensors.