There are many reasons to switch to the updated Sites, including the following:
- Resides within Google Drive: you can create new sites and access your existing sites right within Drive, no going to a different location
- User interface is more intuitive: actions are much more intuitive on the new sites
- ADA Compliance: the new sites works hard to force you to be compliant, which means a limit in some features. It is still the user content that can break this, but the dynamic layouts that adjust to devices, color contrasting within the themes and ease in adding Alt Text to images are game changers from the old sites
- Loss of the page-level sharing: this will only impact a small percentage of people, but for me it was key in having part of a site viewable to the public and part of it private to select people.
- Addition of more features: as it is still developing, there are many features that may not be there...yet. In just the last week one of these appeared (adding a divider, AKA horizontal rule line), so I list this as a con, but it will be less of one as time passes and more features are added.