For practical policy debates, the most important ethical principle is that the pain and suffering—and the joy—of each human being is of equal importance, without regard to who that person is.  Treating some human beings and their concerns as being of lesser importance is the root of much evil in the world.

For those who cannot manage to approach the well-being of all human beings on an equal footing (and of course, this includes almost all of us in our personal dealings), let me recommend this:

At least for public policy purposes, 
on the way to treating the concerns of all human beings as of equal value,
let us treat the concerns of those human beings we treat as least important
as being at least one-hundredth as important
as the concerns of those we treat as most important.

All of that is an introduction for this link to a robust Twitter discussion on the ethics of immigration policy.

Update: I have added a Twitter discussion sparked by this post at the end of the storified tweets from earlier. In that discussion, I call the rule just above, treating foreigners as at least one-hundredth as important as citizens, the tin rule.