If you are looking at the strengths and weaknesses of the most popular open source web platforms (WordPress vs Drupal vs Joomla) the image below might help with your choice.
As you will see WordPress appears to be the most popular and has more off the shelf plug-ins which can significantly lower development time (and cost) for the site owner.
Market share has another important spin-off too, there are probably more people around the world who develop with the product which lowers the support risk for any downstream problems with your site.
Finally Search Engine Optimisation (SEO) is about how well search engines find and display your content in their search listings. To the site owner this translates to the number of free new visitors delivered by search engines to the site. WordPress is better out of the box, but the others can be tweaked to deliver similar results. WordPress just does it naturally (even better with a free plugin) and this again saves the site owner design time and money.
The comparison was kindly provided by Devious Media, and taken from their article here