Jon_King - it's funny, but I just discovered today that LinkedIn has a "skills" section and if you enter a skill, it will tell you about relative growth (presumably based on change in members listing that skill) and total size (presumably total number of members listing that skill)
Drupal size: 14K
Joomla size: 16K
Drupal's growth is higher than Joomla's but not as high as some new entries like Silverstripe (no surprise there, because two years ago there would have been zero people listing Silverstripe as a "skill").
I find this a pretty good measure for answering your question, because it is presumably a measure of professionals who have, at least to some degree, staked their career (or at least career profile) on these apps.
Of course, the results get really interesting when you start looking at "skills" like "candy", "chocolate", "gourmet", "fine jewelry", "wine"... but that would be rather off topic.