I've been a user of the web for business for over 20 years and working on web projects for somewhere around 15 and it still amazes me how you're looking for one piece of information and you stumble on something else to satisfy a question that may have been asked in passing.
Clients and associates ask me why should we use Drupal over Joomla and my pat answer has been that it started with extensive research for a client several years ago when they were looking at deploying mini sites at a fever pitch (thankfully they came to the conclusion they would be diluting their brand by doing so, but that's another conversation).
At the time the big players on the Open Source / Free space were Drupal, Joomla and MovableType. Although on the scale we were looking at, the client would need the enterprise version of MovableType, which is not free, we had implemented a pure blogging site for them already on that platform. In addition there were several in the proprietary realm. Those were dismissed as the cost of entry was too high so I can't even remember their names at this point.
I then focused on the communities and penetration of the remaining options and, at the time MoveableType had the largest share, but it really wasn't a Content Management System, it was a blogging platform. Yes many companies successfully deployed CMS style sites on MT, but in my opinion, you were bending something made for one task into doing another task. Yeah, it works, but if you're starting fresh, why apply duct tape to something that hasn't rolled out yet?
Now I was left with Drupal & Joomla, both of which are still standing strong. At the time the biggest consideration was the ability to host multiple sites on a single platform. That was the final straw. Drupal did it, but Joomla didn't and the decision was made for the client.
I need to know if hitching my horse to this platform would be detrimental to my reputation. I dug further and found that while both had even penetration, my gut told me that Drupal was more robust and had a more vibrant developer community.
Now for the rest of the story. Two different sites BackEndBattles.com and BuiltWith.com are now agreeing that Drupal is more widely accepted than Joomla. Depending upon who you listen to, it's as big as a 5 to 1 ratio.
It's nice to know that years later, your gut was right.