A "top" link is a link that has been the hottest link on all of reddit at some point.
There isn't really an analogous concept on Digg since there is no notion of hottness and links don't rise and fall. Links just appear at the top of the front page at some point.
An interesting comparison would be the last n articles that have been ranked <= 25 for the default sub-reddit mix. At least that would compare the articles a user would see on each frontpage without customization.
Well to answer my own question (close enough anyway) the top 10 submitters of the top 500 submissions this week account for 15.2% of articles, for this month its 15.8%
There isn't really an analogous concept on Digg since there is no notion of hottness and links don't rise and fall. Links just appear at the top of the front page at some point.