Citizenship and Greencardship are pretty much the same thing. The only difference is that you can't vote. (You CAN be drafted)
Also everyone with a greencard is eligable for citizenship after five years so it's a kind of trainee citizenship. Employers don't draw a distinction.
I think it also depends on which country the 'alien' comes from. People always think Germans and English people are intelligent and want them around. Poor nations are not too sexy though. Stereotypes are important.