This kind of thinking is amusing and interesting to debate. (: I don't know who would win. Mexico has a much larger population but Canada is still much wealthier in both GDP and personal income. Mexico is less stable but has more manpower while Canada is developed though with less manpower. The fact that Canada is much larger in area and less dense in population while Mexico is smaller in area with a denser population would probably be better for Canada, though I don't know. Also Canada has many resources. A military strategist would probably know, but I just gave the facts. If I had to choose, i would choose the more developed Canada.
Canada, first of all, has high tech weapons where as Mexico doesn't. Second, I'm sure, knowing how the United States tends to think in times of war, would heavily interfere with that war so it would be two countries in realism lol. But Canada would definetly take the cake
I think Canada because although Canada has a small army it is the best trained. Also none would side with Mexico were country's like the United States. France, Britain and many more would fight with Canada.