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
    https://developer.infusionsoft.com/docs/rest/#!/Account_Info/getAccountProfileUsingGET

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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