They both do the same thing. Except the META tag applies to all anchors on the page. The "nofollow" prevents the link from being followed and therefore prevents page rank to flow.
...control page rank
I'm sure I have read that you should not try and control page rank within your own site by placing "nofollow" on internal links (if someone can confirm this) - if that is your intention. IMO "nofollow" should be used on external links that you do not trust, or links that have been paid for. (Google Webmaster Guidelines on Paid Links [support.google.com])
What is your intention for "signup.php"? If you don't want it indexed then that should be controlled by a "noindex" META tag on the page itself, or blocked by robots.txt.
Ah OK thanks, so there is simply no benefit to other pages on your site, the page rank cannot be 'shaped' (I think is the term I've seen used). So, I can't see any reason to use "nofollow" on internal page links, when a "noindex" on the page itself is probably the desired outcome. You can't control how external sites are linking to your pages.