I prefer to own my US stocks in USD. One of the main reasons to own the CDR versions of these stocks is the currency hedging. Generally, the shorter your time horizon, the larger the need to hedge against currency fluctuations.
Another benefit of these CDRs would be getting the dividends in CAD, especially for someone in retirement that primarily utilizes the CAD to sustain their lifestyle. However, for Amazon, it doesn't pay a dividend, so this portion is irrelevant.
It all boils down to your need to hedge. The longer your time horizon, the more you are capable of withstanding fluctuations in currency and you save the 0.6% annual fee these CDR's charge to hedge currencies.