If you click a link with an empty href you get an error, while clicking a link with # as the href value just jumps you to the top of the page, so the latter is better.
Hm, I thought that was the case, but thought I'd better test it, and it wasn't so—no error message when the href is empty. But still ...
... the real question is—why have a link on your page with either of these? If the link has nowhere to go, it shouldn't be a link.