Correct me if I am wrong, but I believe Air Canada miles have a hard expiration date of 6 years, no? So if you are like me and have accumulated miles over several years, you may run into problems if you are trying to save them for future travel (no lectures please on the wisdom of not spending miles, thanks ). Compare to UA and CO which require more miles for *G status but where you can extend the expiration indefinitely. Also consider how you plan to spend your miles - *A blocking on UA (and potentially CO after the merger) could make it difficult for you to get MP saver awards on *A carriers.

There is more that goes into a program selection decision than ease of status, though that may certainly be an important factor.