Modern sites are advancements in SharePoint to improve User Experiences in Web, Mobiles and provide a "modern experience". Modern Experience is obtained by Sites, Lists, Library experience and pages.
Existing classic sites has a provision to be viewed as Modern. However, it is only limited to list and libraries. So, for any new corporate, starting with SharePoint, the Modern experience is the way to go. Speed, Content rendering Experience, and Responsive design are the key improvements promised.
3 points to think over when you plan for modern sites in your enterprise
Remediating Master Page: Any customization made on top of the existing classic site templates, available in different versions of SharePoint can't be moved as-it-is to modern site templates. The modern site doesn't support custom client-side script executions anymore. SharePoint frameworks become the mandated replacement for custom client-side scripts.
Customizations available in the classic site must be recreated in modern sites using SharePoint Frameworks.
Flattening Hierarchy: When you want Classic sites to Modern sites migration, you must create/move them in "Site Collection" level. Microsoft recommends flat architecture for sites. This is an architectural and design enhancement that MS is bringing inside to improve performance.
The future is going to be Flat Architecture
Pick "Team Site" or "Communication Site" while planning for Modern View: In the modern view, there is only a "Team Site" and "Communication site" template. As of now, Microsoft has only these two templates to create Modern sites. We believe to see more in the future.
Modern Experience is Sites, Lists, Library experience and pages. Microsoft stands committed to improving key features like extensibility, customization, and integration of the modern experience. For any new developments, the Modern experience is the way to go. Existing classic sites would require remediation to embrace full Modern Experience.