Help me push for Drupal !

1. ModX just got to 1.0.2--it didn't even have a 1.0 release until July of this year. That's a relatively young package. Drupal has been going strong for years.
2. The Drupal user community is much, much larger, which means more other people are trying to do the same things you are. (Very important with an open source package.)
3. Drupal is used more widely and more prominently, and therefore has withstood more scrutiny. (whitehouse.gov!)
4. ModX has 623 contrib "extras." Drupal has thousands.
5. If both packages meet your needs, but you know Drupal and don't know another package, that's a reason in itself. Think of the up-front development time that's saved by having a proficient developer from the start. When I think back on the amount of time it took me to learn to do basic things with various software I've used, and then compare that to how quickly I can now get a site off the ground in Drupal, that alone is justification to me.

Unfortunately, it's hard to offer a fair comparison without at least making a prototype site in ModX, but those are the points I'd emphasize if I were trying to convince my boss without any time to get down to the nitty-gritty. Another tactic might be to go over the reported strengths of ModX, and offer a point-by-point on Drupal's strengths in those same areas. (E.g., standards compliance, AJAX, CSS, frontend design flexibility, etc.)

Thanks for your suggestions, those that replied privately and here. It looks like Drupal might lose, but I haven't given up yet!