Thanks so much to the both of you for the detailed and quick responses.  There was some admitted ignorance in my asking my question, because I had had very little contact with either CMS before now--I had done more with Drupal, Joomla!, and Telligent Community Server.  I eliminated Drupal and Joomla! on the basis of wanting to have a single code base based on .NET for server side functionality.  I eliminated Community Server because it was expensive and did not improve function or extensibility any better than open source solutions.

Bottom line is this is what I found since I have reviewed the materials both of you sent:

• Both solutions offer ways to accomplish my goal quite nicely.  Setting up a development environment for Umbraco was slightly easier and natural to do, and I like the way "packages" are done so fluidly.  It reminds me of the neat fluidity of Wordpress in this regard (ability to search for and load packages from the Internet in place as an administrative function).
• After following the instructions Joe has posted on setting up the development environment manually and watching his video going over the code, I had a much better understanding of what was going on and was able to get myself started.  I do wish there were options other than adding all those post-build steps to copy assemblies, etc., however, because my "quote engine"--the back end services infrastructure--makes use of Microsoft Enterprise Patterns for DI and has its own layered architecture, which means about 6-10 more assemblies to copy than in Joe's "WebStore", which is the example I followed.
• As far as theming, mojoPortal seems to have a slight edge for what I am doing although, the XSLT-based model in Umbraco is VERY similar and compatible with the work I am doing on the iPhone application, where I am using XSLT, JTemplate (with JQuery), and other template methods to create HTML on the fly for use in "hybrid" views (HTML local to iPhone and used for display in WebKit while data comes from my back end services mentioned above).  There is some good potential to share XSLT templates, I believe in the future, although this will take more time than I want to spend figuring it out right now.  On this point, mojoPortal still wins for me, because I don't have to create my own theming theory or infrastructure like it seems I have to in Umbraco (unless someone can point me to how I can do it easily without giving my graphic design partner--who knows HTML and typical models for theming like Wordpress and ASP.NET--a heart attack and making HIS job tougher).
• Authentication with the model I am looking for can be implemented either place, but I, at this point, think my intended model for providing applications (iPhone, BlackBerry, and other web based integrations to other sites and widget frameworks) might be more quickly done with mojoPortal by a very slight margin.
• Administration facilities are prettier, cooler, and a bit more functional and extensible--in my quick analysis--with Umbraco.  Like I said, I love the packaging, but I also feel this could be accomplished with mojoPortal by extending it a bit to process zipped files of resources with a manifest XML file somewhere in there telling mojoPortal where to find everything (like the features XML and setup and tear down components).

Thanks again for the quick response!  I do have a question with regard to your statement about not supporting an installable packaging model due to security concerns:

The beauty of ASP.NET and the Application Pool model is that you have so many options for determining how the logged in user can or cannot write to the file system.  It is possible to support dynamically installable features through the configuration of the site, and you can make this "hard" to do, so the installers are forced to have specific privileges.  This would seem to allow the best of both worlds.

I am very interested in what happens with your cloud install on Rackspace, and I read your forum posting on the things that need to be done in order for you to support Azure.  It seems that solutions to those can be found by the time that Azure itself has its model worked out.

Another idea:  would you consider providing REST-based access to each and every feature in a manner that makes it automatic for third party developers to tap into (as a matter of definitions of entities and workflow via attributes, metadata, or configuration without extra coding)?  The reason I ask is that such access (e.g., being able to read/write the blog, forums, threads, or any other "content" based entity equally from REST or web interface) would make integration with other systems and native mobile development automatic.  If there is one thing I would perhaps consider doing as a contribution to the mojoPortal community, that would be the thing.

Finally I really appreciate your approach to building your software as a "labor of love."  This approach to your work will continue to result in spectacular results, and I wish more software providers/developers adopted this point of view.  It makes this a fun job.

Hey, Joe!  Thanks for your reply once again.  I am probably thinking too hard about how to engineer a solution that provides that "downloadable install" kind of convenience just because it looks so cool and seems so convenient despite the risks.

As for the REST API work, I would love to participate in providing that feature, as I will be doing it anyway as part of my current iPhone project.  As I complete my work on this for the first site, I will communicate with you about how it goes and send a proposed design that would work for the larger set of functions on the site.  Take a look at http://blogs.msdn.com/vbertocci/archive/2008/03/10/wcf-rest-at-mix08-the-tale-of-myspace-apis.aspx for my thinking regarding adding REST support to a community site.  MySpace uses this method with WCF to provide basic and OAuth support for authentication. 

Thanks, Joe!  I have already started my own process for this, and I will send you an email, so we can sync up.  I am very excited to work on this!

umbraco? can something that can only be installed to the root and can't be installed to a sub-folder or a virtual directory be called a good system? this this was enough for me to forget umbraco for ever.

how is it compared to the commitment of joe to the project to keep everything perfect?