Both are official. They are built for different sizes of operation, and the honest answer is that plenty of businesses never need the API.
The WhatsApp Business app suits you if
- One person handles the messages
- You reply from a phone, not a computer
- You do not need to message hundreds of people at once
- You do not need to connect it to other systems
The API suits you if
- Several people need to answer the same number
- You want to send approved campaigns to a segmented list
- You want automation to handle routine questions
- You need order data flowing in from your store
- You need reporting beyond what the app shows