“Sign Up” vs “Create an Account”
"Login | Sign up" seems to have more contrast than "Sign in | Sign up" especially for people who don't good at english.
So it is more clear to use "Login | Sign up" than "Sign in | Sign up"
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I agree with @Swain0 and if you want an alternative - which seems to be easier and faster to understand, specially for ESL people(my case) - you can use Login | Register
My vote for "Login | Sign up" because users will know the differences. In first glance, "oh this is for "Login" and this is for Signup".
Also keep both button alongside. or we can give some color variations, so that it will give more visual emphasis.
If you use "Sign in | Sign up", users will have some confusion when he looks at it for the first time.
"Sign" is given more emphasis than "in" & "out" which are in the second position.
if you take this terminology, you need to consider some points:
Also you can find more help from this post: http://www.leemunroe.com/login-vs-signin/
Sign In / Sign Up will increase users' cognitive load because they'll have to concentrate on what it means. Login / Sign up is familiar and they can access without thinking about it.