The biggest advantage modx has over Drupal, Joomla!, etc. is that it simply doesn’t get in the way of your designs. It works along side your design, and doesn’t force you to comply to strict templating structures. Why waste your time to create a beautiful design, to then have it mangled and thrown in a table based layout where its bound not to work as planned (my experience w/ Joomla! has shown me this).
Also, since you are a designer, my guess is that you work hard to ensure that your designs are valid and the code is easy to understand. Like I said before - modx simply doesn’t get in the way. Meaning, you replace a few lines of HTML code with modx snippets, and you have a fully functional template that can be used to edit content as you see fit. The best part - your designs will 9 times out of 10 stay completely valid - I’ve had no troubles keeping my designs valid from Static HTML to modx output... show me a site created in Joomla!, that took an hour (how long it took me to create my first modx template), that validates like that.
You also said that you like Joomla! for it’s vast amount of extensions and plug-ins. I agree, there are thousands of them. But the big question is, how trustworthy are these? Has the developer found all potential security flaws, and made the users aware of them? How well are they supported? Sure, the developer might release a plugin for anyone to download, but if there isn’t any instructions, and no way to contact them, it’s pretty much useless.
Next, I want to tell you that when I started with modx, I had
absolutely zero experience with PHP. None. And I’ve now created 4 sites using it. These sites include features like Wayfinder (with menu’s up to 3 levels deep), Ditto (modx’s top notch blog system), MaxiGallery (built in Photogallery), log in system, and much more. The best part is, I’ve never had to write a single line of PHP code. The documentation floating around here is so superb that just by copying & pasting code where the instructions tell me to, I can create a fully functional blog with an RSS feed and a comments area... it’s been a lifesaver for me. All of the sites I’ve built are on shared servers with upwards of 30 sites on each. I have never once had a problem with the sites being slow or unaccessible. The only time I have is when my hosting company was doing upgrades to the equipment and they had to shut the servers down for a bit.
Let us know if you need more convincing