I would agree with most of the people, US customs are a lot friendlier than Canadians.. But.. No one brings guns or drugs from Canada into the US.. I havent been but Im sure border guards at US/Mexico border are a lot less friendly than Canada/US.
Canadians have to worry about all the drugs and guns coming from US, so right away they are more "on the edge".
I've never had issues with Canadian Customs in airports, but for some reason US customs in Pearson are all dicks..
So if you travel by car rule of thumb: dont be sketchy, bring your girl with you, put white people in the front, dont give customs attitude and you should be fine