My personal feelings about Joomla (and I have tried using) is that it is not terribly user friendly... less intuitive. And by the way DNN is Open Source too...

If I have to say which one I prefer today, I'd have to say I love how SMF Forums sets up a CSM setup with the plugin Portal you can do with it. Here's a great example of a site a friend and I just created for our guild. We took an old theme and revamped it completely. You can't see a whole lot of it unfortunately since we have most everyone locked out other than guildies, but you should be able to see the basics of what it can be like which is very nice looking, super functional, easy to alter, easy to edit, and great access levels too which Joomla is really nasty about.

http://daybreakers.lbpages.com

Here's a screenshot of what it looks like in full bloom:

http://wolverana.com/screenshots/dbsitescreenie1.jpg
http://wolverana.com/screenshots/dbsitescreenie2.jpg

Very slick, quite a few themes you can choose from, if you have some coding experience to find the image areas in the files it's not too hard to alter the graphics and there's quite a few layouts out there to choose from so what I did was search for the layout type I wanted and then altered the images to our guild's tastes. Worked out pretty good I think, we just got it finished a couple days ago and took us about 3 days to set it all up skin and all.

Joomla, Drupal and Wordpress can all forget it, this one beats them all as far as I am concerned having worked with all of them quite a bit.

A very nice CMS system I have been using on some new projects is Pixie http://www.getpixie.co.uk/. It is very functional and lightweight.
Easy to setup and manage.