There are benefits and drawbacks to both approaches. Largely it depends on how far you are willing to go in either direction.
Static Files
These files are not manipulated themselves by MODX and removed from security protocols that govern resources, making them faster to load, in general. The primary benefit to sticking with static files comes down to flexibility with external editing. Since the files themselves are typically uploaded, it doesn't matter what tools you use to edit them. My personal issue with this is that I consider Media Sources a security hole (not because they actually are, but they make me feel uneasy, nonetheless). I lean strongly toward database managed security, and that bias is reflected throughout this post.
MODX Document Resources
These files are in the database, and have all of the benefits that are associated with Resources. TVs, Custom Forms, MODX Tags, Snippets, etc. Anything that you can do with a MODX Document or Resource,, you may do with your CSS. Editing may still be performed with an external editor, but generally occurs via cut & paste, if using an external editor. A big bonus here is the MODX Parsing as you can apply colors or specific rules via a TV or System Setting (even User Setting). Another often overlooked possibility is using a Template (as you do not have to). I contend that the greatest benefit is database security.
As MODX Document Fragments
This is the approach that I personally use, via RO.IDEs (link in my sig). The idea here is to have the fastest editing while still maintaining flexibility and power of MODX parsing. I can find and edit any style faster than with any exernal editor. I can also search for class names, ids, or even specific rules using the Search Resources tool in the manager. The drawback here is extra resources and if not performed correctly, could increase the load of your page. However, the extra resources are literally as many or as few as you want. On my site, my CSS comprises 400+ micro-resources. I could just as easily do it in 4, but I need the extra resources.
Turning off a single or group of styles takes literally the click of a button. Moving styles is literally drag-and-drop. This makes debugging CSS a dream for me. Each segment of my CSS is as protected or free as I want it to be without having to worry about Media Sources, or .htaccess, etc.
Finally, I have a front-end AJAX editor for my CSS that blows external IDEs out of the water for managing my CSS.
As MODX Static Resources
This provides a good blend of MODX Parsing and external editor support. But you may include MODX Tags and they will be parsed. The only real limitation is that the object code is not actually in the database. While this is a big deal to me, others consider this not a big deal. Additionally, while it provides database managed security, the static file is still accessible if you either a) want it to be, or b) are not careful. (There's that bias again).
[ed. note: fuzzicallogic last edited this post 14 years, 1 month ago.]