It would depend on where you received your medical education and training from, and how much you've incurred in student debt, at least for me. As a med student in the states, if I go into surgery, I would like to practice in the states, at least until my loans are paid off, since I would pay less in taxes, and earn more.
If I were going to medical school in Canada, then I would be more inclined to stay there. Less debt, and medical malpractice is not such a big issue up there, which means you would not be paying a small fortune in malpractice insurance.
In terms of quality of life, you'd have to be more specific. For me, things like education, cost of living, public transportation, health care etc., are important. The only thing where Canada seems to have an edge is public transport. I see too many idiots on SUVs here. But if you are middle class or above, the states can give you access to great schools, and state of the art medical care.
If you live in places like texas, the cost of living will also be dirt cheap. Many places in the States also have infinitely more plesant weather than the great white north.
To me, it is advantage U.S. Someone else could have different priorities and come to an entirely different conclusion, and that's fine. There's no right or wrong answers here.