Neither. The Digg API is free to use.
At one time, the Digg API license permitted only non-commercial
apps. That generated some interesting discussion about what is and
is not a commercial app:
https://groups.google.com/group/diggapi/browse_thread/thread/1961b215b78888ca/0e6a9f4e1d4972f8
In 2009, we removed most of the commercial limitations:
http://about.digg.com/blog/new-digg-api-features-and-more-developer-control
Here's the current API license:
http://developers.digg.com/agreement
Note that while there are few restrictions on the use of the API,
there are restrictions on use of the Digg brand: