This is not properly an answer to your question, but rather couple of additional reflections.
First off: it's not clear to me if you are developing your own system or if you are simply planning to adopt an existing system to do your own site. Of course this changes completely what the answer should be.
Second: some elements you should - IMO - consider:
CCK has been ported to core in Drupal 7, so - if you are after writing your own CMS - you you might wish to checkout a copy of D7 from the CVS repository and see if the new code could work better for you (being in the core has been cleaned up of a lot of stuff and integrated more tightly and more efficiently with the inner working of the system).
Drupal vs. DotNetNuke from a maturity point of view is a nobrainer, really. Drupal is solid, widely adopted, has a huge community and is constantly winning awards for being one of the best CMS out there. Even the whitehouse is now using it to run its own website, and you can find much more case studies on the drupal site if you are curious to know more about the way functionality was achieved. I think you should consider this when estimating how big is you significant advantage because of the .NET language. Chances are that if you want to use the existing CMS for you to create a site, the advantage of the known programming language will be annihilated by the fact you will have to implement "by hand" a lot of functionalities that you would have got out-of-the-box with Drupal or one of the thousands of contributed modules.
You mentioned that you want it Open Source. Drupal is licensed under GPL 2. All code and documentation is available and free-as-in-speech. DNN - you can check it out by yourself - offers various products of which only the base one is free. Moreover, the free version is stripped of key-features that are probably essential on most serious project such as access to complete documentation, vulnerability database, health monitoring, fine permission management, etc... This business model / licensing scheme is often referred to as crippleware:
Crippleware programs are usually freeware versions of computer programs that lack the most advanced (or in some cases, even crucial) features of the original program. Crippleware versions are made available in order to increase the popularity of the full program without giving it away free.
[For a different take on DNN licensing scheme, read the upvoted comment of Ian Robinson here below]
- Also, Drupal is developed in PHP, a cross-platform free (as in freedom) language: this means you can host a Drupal site on a server of your liking. DNN uses a close sourced proprietary language (yes I know about mono!).
EDIT (poster clarified he wants to use an existing CMS for building a site)
Probably the first point of what above is irrelevant to you then.
If you are going to adopt a CMS for a website and you want to put your hands on its code, IMO sensible choice is reduced to Drupal or Joomla (and in some regards WordPress). Those are the true leaders in the FLOSS CMS.
Joomla has a lower learning curve as for using it as a site builder / content editor. The UI is quite intuitive and well laid out. Drupal is less well designed in this regards (although a lot of improvements are about to come out with Drupal 7) but wins hands down when it comes to developing new functinality for it: the code is cleaner and more rational. The internal API's are very powerful and very well designed.
Drupal is not the easiest framework to learn to develop for. In assessing the difficulties your team might face, I would say that learning PHP would be one third of the job, while learning "the drupal way" would be the remaining 2 thirds. Last week another user has posted a couple of good questions for Drupal beginners. Some of the advices given there might be useful to you too. Have a look here and here.
HTH!