look at it this way: since they're asking, I have to assume they're not too tech-savvy, so Drupal is out of the question. So you can narrow it to both CMS you're mentioning. WP is hands down easier, faster and more expandable than Joomla. Plus, for every Joomla site you'll have 100 WP sites, so, unless the vast majority is wrong and clueless, there you have your answer.
