Personally I believe .com still beats country-specific domains. But, you never know when Google change this. Buy both the extensions if they are available, otherwise buy available one
First, you have to know what market you are targeting. Are you looking for canadians only or may you want to expand later on? If it's Canada only, you go with the .ca, as most people use google canada and your domain will rank better than a similar .com domain. But as other said, you "have!" to buy the .com, it's 10$/year and it will save you a lot a trouble later on if you choose to expand.
.ca definitely as you are targeting Canada. There is no reason to go for .com if your main market is Canada.