I've had this dilemma before. Buy both, use the .ca if it's primarily Canadian customers and redirect the .com to the .ca This will identify with Canadians better and if they type in .com by accident then they will be redirected. If you expand to US, then use the .com for that and keep .ca for Canadian customers. You will also want to host in Canada to improve Canadian rankings, this means a Canadian IP not just a host that says they are based in Canada. Also, .ca extensions are supposed to rank better in Canadian indexes. Keep those things in mind.