The main difference between the API v1 and v2 is the way that the content is structured.
The content structure for API v1 is more complex and meant to be used with one of our development kits.
The content structure for API v2 has been simplified to make retrieving your content more succinct and to reduce the reliability on the development kits.
The next important distinction is that the API v2 provides default empty values for fields that are left empty.
If a content author doesn't fill in a given field, the field will not appear on the API v1 response. For the API v2, it will appear as either an empty string, an empty array, or a null value.
The last difference between the two is for when a field is removed from a Custom Type. With the API v1, if the field previously had content, it will continue to appear in the API response. With the API v2, no matter the field's previous content, it will no longer appear in the API response.