Integration Infusion soft

We need to help our Customer.
From our Application to connect our Client infusion soft account.
Which is the best way to do?

  1. REST API
    Keap REST API

or

2)XML RPC
https://developer.infusionsoft.com/docs/xml-rpc/

What is the difference between above two?

How to add multiple app account in infusionsoft?

We have one app account
https://by693.infusionsoft.com/

See the other thread for response.

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