I've noticed that some sites feature register button (or form) more prominently than login button (or form), such as Dropbox and Facebook, whereas some other sites are the opposite. And also, I've noticed that in some mobile apps like Facebook app, the login button is more prominent than register button.
From user experience perspective (and business perspective), which one should be more prominent? Will new user feels alienated by greeted with login form first? Will returning user get annoyed for the need to click on the "Returning user? Click here to login!" button each time?