I almost exclusively buy AAA beef because the price difference for Canada Prime is significant. But for someone who can tell the difference between USDA Prime and AAA they should buy Canada Prime while in Canada.
Canada Prime and USDA Prime have the same requirements for marbling (literally exactly the same, Canada adopted the US standard for marbling in the '80s), and the Canadian standard has additional requirements for animal age, fat colour, firmness, and meat colour.
AAA beef has the same marbling requirements as USDA choice, and a similar list of other additional requirements.
There is also a slight technical difference in the different grading categories between the countries. USDA standard allows an exceptional category to offset a weak one, whereas in Caanda beef will be scored at its lowest category.
As an example, a steer that had some characteristics prime, some choice, and one select might be scored choice in the US, with the prime characteristics averaging out the select ones. In Canada that would be a AA steer (equivalent to select) because of the lowest category.
Basically, in Canada any Canada Prime meat would grade USDA Prime, any AAA would grade USDA choice, and any AA would grade USDA select. The opposite isn't quite true - some USDA Prime would grade AAA and some USDA choice would grade AA for various reasons. Like I said earlier - I'm a picky buyer of AAA meat, and I can almost always select a package that (imo) would grade USDA Prime or close to it at substantial savings from buying Canada Prime beef.