Sign In — Sign Up / Login — Register
To give your users a faster joining experience into your system you need to clearly communicate where a first time visiting user & registered user should go. Facebook, Reddit uses Login, whereas Google, Twitter, Bank of America and LinkedIn use Sign in.
To avoid confusion for users its highly recommended that you avoid using labels Sign In — Sign Up because these contain same word “Sign” which takes much cognitive effort to differentiate. So using Login — Register is recommended but Log in is more of a technical term which is usually used in admin portals like WordPress.
There are better alternate words to notify the first time & registered users. Again, It totally depends on your target audience and their understanding of services provided by the company website they have visited.
First time visiting users:
- Join [Name of company] —Usually used if your company is a Club/Community
- Create Account
- Subscribe — Used for subscription based services like Newsletters
- Already a member?
- Sign In
Any combination of labels from above lists will give better clarity to users thus helping them navigate and get the results that they are expecting.