Drupal vs. Dot Net Nuke vs. Mambo?

I have been searching for a CMS for the non-profit org that I work with. We're about 40 people at our office now, planning to continue growing, working from 3 locations within the city. We've got a mix of ages, cultures, and levels of "IT IQ"....

As you can tell from the post subject, I'm looking at Drupal, Dot Net Nuke, and Mambo. If anyone could give me some suggestions/opinions as to what they think is best, I'd really appreciate it. I've been reading the specs and other info on each of the systems, but just can't make the decision.

Some things I have to consider in choosing the CMS are:
1. Ease of use for multiple users/contributors.
Our site needs to be something that contributors (with limited/varying computer skills) can easily update...things like dates/events, information, news, pictures, video.

2. Not a steep learning curve.
We have a huge range of technological comfort and skill levels. I'm also pretty new to CMS stuff, etc, and need something without a huge learning curve. We also need to get the site running quickly. We have a site now, but it is disorganized and not even all the same visual design. (We put our current site on hold because we wanted to migrate all our info to something new, using an open source CMS.) This boils down to --> we need to get things done quickly and don't want to get stuck for several more months learning another CMS.

3. Easy to use RSS feeds

4. Ability to have a creative design that isn't too "boxy" but will also lay out our information in a helpful, easy to follow way

5. Ability to offer different permission levels to different contributors. Also, ability to limit access to certain sensitive areas on the site to different readers.

6. Ability to create a sense of community among visitors and staff users of the site.

Those are all the questions coming to mind right now. There are probably other questionsI haven't thought of yet. It'd be great if anyone can tell me anything about each of these systems, why one might be better than the other for different things, etc. Of course I'm still doing my own research on them, but I figured there would be some experts in here who could point me to the right "selling" points.

Thanks!