Hi Vangogh,
I was just reading something about the word portion of the question you're
asking... but not with "click here"; instead it was a study between "submit" and "sign up" ...
This might be a part of the anwer for you; but I don't know if there's a test comparing "click here" with "sign up"
You'll find the short article here:
http://www.jamesbrausch.com/?p=57
Maybe you could get him to test click here for you!
Good Luck
Lizzie