I am going to be putting together a proposal for a prospective client who is presently using Concrete5. I am not at all keen about getting involved with PHP and would much prefer to stick with DNN. Can anyone point me to any resources that make a convincing argument for DNN against some of its PHP competitors (a direct comparison to Concrete5 would be really nice). Or can anyone based on their own experience tell me whether I'm wise to shy away from Concrete5. Maybe I should just bite the bullet and learn Concrete5. Any thoughts?

I didn't select Concrete5. They have an existing website which is pretty basic for which they'd like to add some new functionality (mobile friendly, members only content, online reservations, etc.). I know I can do all of those things in DNN. In Concrete5, I presume it is possible but it may be a pain. I also don't really know how good a CMS Concrete5 is. Am I building on a solid base if I use Concrete5? I've used DNN for years and know that it works well and is well supported. Can I say the same for Concrete5? I also hate PHP (but that's my problem). However, if I am going to convince them to use something other than Concrete5 (DNN, for instance), I've got to have some compelling reasons.

I've used both, but usually would lean towards DNN because that is what I am more comfortable with and I know I can accomplish a lot with the platform. So, is the cost of you learning PHP/C5 greater than migrating them out of their current platform? It sounds to me like you are guiding a decision based primarily on your comfort level and not the capabilities. Why not demo DNN and show them the tools at their disposal and gauge their comfort level? You also really have to consider the other conditions that come with a wholesale platform change, like hosting for instance. 

Incidentally, I did like C5 the last time I looked at it and would consider for a PHP based CMS. Not before WordPress of course, but I would consider it before Joomla or Drupal for sure. It was kind of "DNNish" in a way.