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.