On the whole, a Canadian citizen will be more promising in the U.S. Canadians speak English which is close to Standard American English; the cultures are more similar, and the level of poverty is much lower, meaning they're less likely to become dependent on social programs. If usefulness is measured in ability to contribute to the productivity of the nation as a whole, then Canadians are a better bet (knowing nothing else about the individual).
All else being equal though, I don't see how you could make the claim that the U.S. is engaged in favouritism between equally-promising Canadian and Mexican potential immigrants. Mexicans and Canadians both (technically) need to get permits to work in the U.S.
The Québécois were estimated in 2011 to be about 43% functionally franco-anglo bilingual; seems they're becoming more so year over year, the estimate was about 41% in 2006. About 8% of Québécois have English as their mother tongue. 5% of Québec doesn't even speak French, granted some of those will be allophones and illiterates.
There are plenty of Mexican illegal immigrants in Canada, probably helping with our huge pulse crops.
Anyway, there's no point in enhancing the productivity of U.S. domestic agriculture. It is incredibly productive and food is incredibly cheap at the wholesale level. If it becomes a bit more expensive at the wholesale level, I doubt that will shift the consumer price all that much. The people who would have more trouble affording food would also have higher mean income, since they would be more likely to be employed.
All else being equal though, I don't see how you could make the claim that the U.S. is engaged in favouritism between equally-promising Canadian and Mexican potential immigrants. Mexicans and Canadians both (technically) need to get permits to work in the U.S.