Unfortunately every province in Canada has different rules on out of province coverage and how long you can be away. But getting back to Almot's and the OP's questions. I would not consider SP to be good insurance. The clinics are very basic and often not clean. IMSS coverage is not that expensive. Keep in kind that if you get RP status you cannot have a Canadian car with Canadian plates. You will not lose your Canadian tax status. I would suggest that when you decide where you want to retire visit the local IMSS facilities and see for yourself if it would suffice. Even in the Chapala area where the hospitals are better many X-pats do not use the service. In smaller communities the clinics are very basic. BUT many of the Drs. are recent grads who are very savvy. The problem is they are not equipped to do anything more than a stitch or a cast. The Kino clinic does not even have an EKG machine nor any x-ray equipment. Someone in one of the other posts made reference to freeloading Gringos. Seguro Popular was really not intended for foreigners. But Mexico kindly looked the other way. I would suggest that using that service is "freeloading". BTW: Some foreigners buy catastrophic insurance or x-pat insurance with a big deductible. They just pay for minor problems.
Moisheh