Virtually none. While Google (for instance) prefers "sign in", Facebook likes "login" better. And so on.
However, as a Software Engineer myself, I always recommend that programmers and designers use "log in" or "login", in order to make a clear distinction between "Sign up" and "Login", which sometimes can otherwise be confused.
English is not my first language, in case I made mistakes in my writing. Nonetheless, I'm 99.99% certain about my answer. Let's see other responses, though.