Google Contacts API vs People API
I found the difference between the 2 APIs and it has nothing to do with the fact that one of the accounts is connected to Google+ or not
People API will only fetch the contacts that are under the account "My contacts" contacts, while Contacts API will also fetch the other contacts lists ("Other contacts", "Most contacts", ...)
The People API is the newer version.
From Google APIs related to People API page:
The Google Contacts API will be deprecated in the future. The People API is the recommended replacement for apps that read contact data.
It is also important to note that the People API does in fact allow the developer to use "other contacts" just like Contacts API did. In addition, when migrating you should not have to re-ask for user consent to the following:
- https://www.google.com/m8/feeds
- https://www.googleapis.com/auth/contacts
- https://www.googleapis.com/auth/contacts.readonly
Please see these links for further reading:
https://developers.google.com/people/related-apis (Contacts API vs People API) https://developers.google.com/people (Intro to People API)
I basically just copy/pasted most of this stuff but hopefully, some people find this helpful as I too was curious to find these answers :)