WordPress templates are always files, and you can lose a lot of time because you don't know which template file is being used to generate a given page. Several holes in my wall (and head) are branded by WordPress because of this "feature": it may save time while setting up a site, but it always costs time when you have to troubleshoot someone else's work and you can't find anything.
MODX stores template HTML in the database OR on the file system, and for each page, you can select a template from the dropdown list (it's what's available for WordPress *pages* -- posts do not have the ability to select individual templates). As Bob mentioned: you can create a template as a "static" resource, i.e. save it on the file system for easier editing, but as leelondon warns, you need to watch out for caching. MODX has a thorough caching engine. Out of the box it uses a fraction of the function calls to generate a page as WordPress does, and once you start using MODX caching, it quickly leaves WordPress in the dust (yes, even with WP's caching plugins -- I did an A/B test on this in my MODX vs. WP book that's due out this year).
Re the platform: check out the requirements: http://rtfm.modx.com/display/revolution20/Server+Requirements
One of the things that distinguishes MODX is its freedom for front-end designers. It is the easiest CMS I know of to work with as a designer: you can literally take any
HTML and import it into MODX. The templating practices implemented by WordPress, Drupal, and Joomla are pretty much same same same: they use a maze of PHP files, so you end up having to debug them, and good luck if you're using a bunch of plugins that may have molested your template dynamically at runtime. Designers don't want to touch PHP themes because they've got PHP in them (sometimes lots
of PHP). Developers don't want to touch them because they've got HTML in them. MODX templates can get messy too, but generally they are clean and any designer quickly understands the placeholder syntax. I worked on one project where we needed to import working HTML wireframes into a CMS. The backend was already in Drupal, but the lowest bid I got for the task in Drupal was $10,000. The same task cost $300 in MODX. Pretty unbelievable. Anyway, I'm ranting.