GPAs in Canada are higher because they're calculated differently, but it's still a 4.0 scale. That means that even if you apply as a citizen, they'll still only see your GPA as-is, which can be extremely prohibitive considering you generally need a 3.85+ to be highly competitive. It would also depend on which province you'll have residence in, though.
As for practicing, you're free to take the USMLE while studying in Canada (plenty of my classmates are planning on it), so you can apply to residency programs in the US. In terms of competitiveness I'm not sure how much of a disadvantage you'd be at, but no matter where you apply in the States you won't have been able to do any rotations there which can't help. I'm not quite sure how the matching system works in the US, but here US MDs are placed in the same pool as Canadian MDs so there theoretically isn't a huge disadvantage, with the caveat that I mentioned of not having any connections at the hospitals you're applying to.
edit: Also keep in mind that the average successful Canadian applicant takes several cycles to get in, so don't hold your breath if you're only going to be applying once to both countries.