"Certified Angus Beef" was started out by the Angus Beef Association as an ad campaign. Now to qualify as CAB all they need to be is 50% or more black on the hide. Seems a tad less official then before.
When CAB questions come up it reminds me of the Jack in the Box commercial for their sirloin burger versus the Angus burgers.
I think you have to take in the factors of the rancher, the feed, the age, the cut, etc...
If I was going in blind, I would probably buy into the CAB just as it does at the least have a certified grade and generally holds a higher quality in one shape or form.