For what it's worth (source Cdn Beef Grading Agency)
Canada Prime grade is virtually identical to the U.S.D.A. Prime grade
Canada AAA grade is virtually identical to the U.S.D.A. Choice grade.
Canada AA grade has comparable marbling to the U.S.D.A. Select grade.
The Canada A grade is unique to Canada in that there is less marbling but all other quality attributes are still present. This particular grade is well suited to those consumers now wishing to limit their level of fat intake while still wishing to enjoy the eating experience of high quality grain-fed beef.
The only other difference I saw between the two grading systems was that the Canadian grade (prime, AAA, AA) does not allow dark coloured meat, yellow fat, older animals, or other off-quality characteristics. The US grading system does.
I copied this information from the CBGA web site.
I suppose that there are people in the trade that have more accurate info or details.