Hands down WordPress is a better choice when it comes to customization, web accessibility, ability to SEO optimize all content, and content management. Drupal is the second best option but it will be more expensive to manage and far less user friendly. Joomla is at the bottom of the barrel, on top of the advantages mentioned already Joomla is not well supported by professional web development agencies.
Drupal 7 will be supported until November, 2022, meaning Drupal 6 will become obsolete. But the platform itself is here to stay, especially for highly complex enterprise level websites.
This depends on your goals—if you have a very small budget, don’t drive a lot of business from your website, and don’t need it to handle any complicated functions, DIY with a good WordPress template might be right for you. But if you rely on the site for anything other than proving your company is online, or work in a competitive market, a web design agency is the right choice.
This entirely depends—a skilled Drupal developer can create a custom backend that makes it very easy for laypeople on your team to maintain the site, even without that developer’s help. But out of the box, Drupal can be archaic.
Out of the box, Drupal might look more secure, but with the right hosting, plugins, and best practices from your team in running the site, WordPress stands shoulder to shoulder with Drupal.
While both WordPress and Drupal CMS platforms are products, web design and development is absolutely a service and your developer should be a partner. In general, a web design agency that is selling “productized” websites is pitching you a cookie cutter website that is not likely to address your business or brand needs, which will only lead to you needing a redesign in a couple years.
If you have both Drupal sites and WordPress sites among your properties, even for one brand, not to worry. Many of the biggest companies do it. There’s one caveat: it is more learning intensive to maintain both kinds of sites for whoever is going to do it. If you have dedicated teams for each site, or a developer who can handle both for ongoing maintenance, then no problem.