That little aside about "log in" vs "login" irritates me because there is a correct answer. "login" is a noun, while "log in" is a verb phrase. They are two separate things. The button to log a person into the system is a command (a verb), thus it should be "Log In", not "Login".
While what you say is true, what matters more is the UI experience. Some people will naturally conflate "login" and "log in", even if they shouldn't.
Also, there are other factors - fonts, monitor dpi, and other factors may make it more difficult to discern between "login" and "log in" on a button, for instance.
