Since I have known MODx for almost 4 years and built a bunch of websites with it, and now have a good grasp of what Drupal theming involves having actually build one on a complex project, I’d say the approach to theming is radically different.
MODx does not get in your way or force you into learning complex naming convention of template inheritance concepts. Working with MODx is like working static websites for a designer, with the benefits of making it dynamic with a few easy tags and snippet calls. With MODx you never loose the global perspective of what you’re doing because MODx is template centric. Template variables, snippet calls or chunks are added to a single template. Yes snippets can have their own templates stored in chunks (like wayfinder or Ditto) but the key pivotal element is the template.
Drupal is, too me, feature centric. When you use PHPTemplate, Drupal has a very structured approach, with naming conventions, a hierarchy, inheritance, overrides... when you look at a page generated by Drupal with the Devel Theme module you can see it refers to a LOT of templates. Templating is "atomized" into a lot of small bits, it’s the result of a very rational/analytical architecture. It’s also a result of Drupal being modular to the extreme, when like me you end up with 100 modules for a website, it also often means many more templates. It ends up being quite bloated. The design process is (to me) quite painful, even when you get the logic behind it. It takes a lot of learning, and makes designing 3 times longer than the same design with MODx. Also, maintenance is made harder with the sheer number of overrides any complex website is bound to have... I document everything with Drupal for that very reason. Now I am talking templating, and Drupal also has strengths which explain why it’s so widely adopted : a well documented API, a lot of contributed modules, from what nylauster tells me Forms API is unparalleled... etc.
This being said, any time MODx has the required features (I am no developper and the client doesn’t always want to / can’t always fund development of a missing MODx module, snippet or plugin) I’ll pick it over Drupal no sweat
I think we’re entering a very interresting period for the CMS market... Custom content types and/or variables have become a commodity : MODx always had TVs, Drupal has CCK, Joomla now has K2 and even WordPress has a couple of plugins to handle it.
modx Revolution can really change the game if it reaches a critical mass to get enough developpers on board, therefore key addons released, Drupal 7 is going towards a tremendous effort towards better usability (thanks to Mark Boulton) and maybe also a new theming layer (?) and also a new DB layer not unlike MODx xPDO from what I have read, ExpressionEngine is rebuilt using CodeIgniter - enjoying a community of developpers familiar with CodeIgniter will boost EE addon production, I bet...etc.