definitely the second, because you want to highlight the Sign Up button for new users. already registered users are motivated enough to find the Sign In button even if it’s not the primary call to action on the page.

writen by Luca Restagno (ikoichi on Twitter)
I think the “Sign Up” makes more sense cause that’s what we need.
Additionally, give a small login page link in the signup page.

writen by WBE User
Yes, ‘Sign Up’ should be preselected by default for new users. Even better yet would be if you can distinguish between new and returning users (e.g. by cookie) and have the UX behave differently accordingly.

writen by Stephanie Preuthen