This is somewhat oversimplified but interesting infographic comparing the three opensource platforms.
Fascination Design uses both WordPress and Drupal based on the client’s needs. Joomla is also a good platform but it’s really an in-between solution which is why we chose to learn the two extremes. WordPress is easier to get started but not quite as robust as Drupal. And Drupal handles more customization and larger e-commerce sites better.
Quick note, there is also a difference between WordPress.com and WordPress.org. I’ll be doing a review and breakdown of that at a later date.