To have a button that, when clicked, takes the user to another page, you could just have it submit to a script that sends back an HTTP 301 header with the location of the page to redirect to, but why not just use an actual link and use CSS to style it however you want it to look?
You could use CSS to make the 'register' link look like a button, by using colors, borders, etc.
Or have it submit to a script that redirects to a login page. But honestly, I don't se any reason why they both have to be/look like buttons; IMO buttons should look like buttons and links should look like links.
Well, if I understood it right -- to make one button work as submit button and another one as a link, make two <form>...</form> sections with each button inside, one for submission and one for the link.