I'm a dual US/UK citizen living in the UK for a while. Last time I
returned from the US I had a peculiar conversation with the Delta
passport-checking person at the bag drop (I'd already checked in
online):

While living overseas, usually on leaving the US for the airline
passport check I show both:

* My US passport, because the INA requires US citizens to depart from
the US bearing a valid US passport.

* My UK passport, because I don't have a return ticket from the UK so
the airline can see that's not an immigration control issue as I have
right of abode there.

In the past, they've glanced at both and selected one and done whatever
they do with it.

The Delta guy was most anxious that I pick a passport for him though was
unable to say anything coherent (or true) about on what basis I might do
so. Is there in any way a correct answer? Can anyone explain what that
was about? Maybe there are legal effects of my telling him to use the US
or UK one?

-- Mark