I usually take a slightly different tack because the technical level of people I usually am dealing with is lower. I tend to be pitching to decision makers and not tech people. A bit of my standard answer is below.

Wix and Squarespace are proprietary systems meant for unsophisticated users who want to build their own websites quickly and easily. While they are good for that specific use case, they do not offer any way to move beyond that if your needs arise. Since they are proprietary closed systems if you need something more advanced at some point your only option is to start over.

WordPress is an Open Source CMS that allows much more freedom. It is not quite as simple to setup and create a new site but if you are talking to me then you are not looking to build it yourself so that is really a non-issue. The main benefit of WordPress is freedom. You can host it on virtually any decent web hosting service and since it uses PHP and MySQL you can have virtually any developer take over a project without problem.

I believe in open source because of that freedom. It is good for me as a developer and it is good for my clients. If something were to happen to me or my company you would have no problem finding another qualified WordPress developer to take over the site in a totally seamless fashion. There would be no need to start from scratch.

Additionally the extensible nature of WordPress means that no matter what your future needs, WordPress can handle it. Adding things like e-commerce and custom quoting systems are just two examples of advanced solution's that I have added to WordPress sites years after they were first built.

WordPress is used by tiny one person businesses all the way up to major websites like the NY Times and I think it is right for this project as well.

So many choices for CMSs these days. So then what do you choose if speed, security and customization are key? Headless for one. Consuming your own APIs for content is absolute key. It makes designing pages in the front-end a breeze. Leaving Ghost and Cockpit. If I then looked at the footprint and impact on server load, Cockpit definitely wins that battle.

if main concern is licensing cost, it is best to take the open source route (WordPress or Drupal), because you don't need to pay any license fee for using these software. As others might have stated, Adobe Experience Manager license fee could be costly and that doesn't even include development costs.

In WordPress ecosystem, plugin and theme developers are competing with each other to provide the best free plugins or themes for public use. Further, developers can provide support or more features added to plugins or themes when you pay a small fee for license.

The downside, although rarely happens, when there are conflicting themes/plugins it can cause your site to break. Another problem, when it comes to security, most often that you are at the pity of themes/plugins developers.

In Drupal ecosystem, its developers hold the famous principle, "not to reinvent the wheel" which is the reason why you won't find as many modules or themes. Drupal developers rarely competing with each other but instead they (even non-programmers) will work together to improve existing themes or modules. Obviously one module will not meet everyone's criteria, therefore in Drupal they have 'hooks'. A Drupal 'hook' can extend Drupal core's or module's functionality to meet your requirements.

I would tend to use Drupal but I think it is only a matter of preference. I admit that building on WordPress is easier while building on Drupal will require a lot of research and experience.