A lot depends upon what you want out of life.
Canada will offer you a place where the standard of living is high, the crime rate low, and the climate healthy, the life expectancy high, and the people generally friendly but without being over the top. It does get very cold, but the summers are beautiful, and the culture is varied.
America will offer you a place where the standard of living is relatively high but falling, the crime rate generally high, the climate not so healthy, the people not always so friendly and often over the top. But high national security and a patriotic feel. America has a wide range of climates, suffers from tornadoes and hurricanes, can be heavily polluted, and has ads every few minutes on the TV (very short attention span)... Oh and it currently has GWB.
You decide :)
And before anyone complains, I am not Canadian, just honest :)
PS. all these things vary to a large degree on the city/state/area you will live.