REST vs SOAP Desktop Add-in

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I would like to create a custom object from a contact workspace desktop add-in (e.g. a button on the workspace, that when clicked creates a custom object).

As far as I can see, the add-in api does not have access to create custom object from a contact workspace (Correct me if I'm wrong here).

From the docs, the solution appears to be to use the SOAP api. I am wondering if it's considered better practice to use the SOAP api in a workspace add-in than to use REST. Or is it just a case of the REST api being created after the add-in documentation.