Just to clarify, Angus, or Black Angus, is a breed of cattle. Certified Angus is a trade mark, and is not, by itself, a quality mark. It is really no different than other marketing names. The Angus association has done a good job of marketing, resulting in higher prices, and I think they do some policing. Hanging in a packing house, there is little distinguishible difference, although a good tagger can pick them out. USDA grading is a quality designation.
I doubt is anyone could tell the difference in a steak after cooking. As many have said, the three most important factors in a quality steak is aging, marbling, and seasoning.
If you want the best steak possible, do what the best steak houses do. Buy whole strips or rib eyes. Pick the one with the largest eye and the most marbling, then store it in the frig for 6 or so weeks in its original Cryovac packaging.