Depends on your application and requirements.

Both frameworks have their Pro's and fewer and fewer Con's with each revision

DNN is generally a more flexible framework, Supporting multiple portals out of the Box. Strong commercialization possibibilities with BSD license but needs tweaking for compliance.    .NET and MS SQL

Joomla has some very nice features too and can be extended in different ways.  Generally  speaking Joomla has a lower cost to operate (hosting) and lower cost for modules.  Strong push is to keep it and all derivative works free / GPL/GNU license.

I would evaluate your skill set, development budget, tools and application and decide.  Test drive both and report back a subjective evaluation based on your findings.