I've used MM.Net since it came out (actually before) and I've written a custom module for DNN (DotNetNuke). DNN's real advantage is in it's content management. It is great if you need to give your users the ability to edit the content of the site through the site. It's hard to compare it to MM.Net since they have different purposes. In fact I'm sure they could work well together since they don't have a lot in common.

MM.Net is a set of classes that help you follow some best practices when it comes to separation of UI from Biz logic. IMO, It's real strength is in it's business object classes. Also in the UI it has a nice clean way of allowing two-way data binding that I don't think you'll see in DNN. IMO, it seems to support Windows Forms a little more that ASP.Net but I may be wrong since I've never used it for a Windows Forms app. I've wished it had more controls in ASP.Net but I found it fairly easy to create those controls myself. It's nice to be able to look at the source code for the controls it does have and use those examples to create the controls I needed.

You might consider listening to the this .Net Rocks show: http://www.dotnetrocks.com/default.aspx?showID=199

It talks about a new ASP.Net framework and mentions some other ones I've never used.