to echo the above post, it is US law to present yourself as a USA citizen when entering the country. i'm sure in practice this isn't such a big deal, but a friend was once told sternly that she "should really get a US passport" as she was born in NJ and it says so on her canadian passport.

personally I use my US passport only for going to the US and my Canadian for all other travels.

the only exception is that i will use my US passport when going to Israel and Canadian to other middle eastern countries.