My customer now wants to communicate with an external site. I am able to choose to communicate with that service either through REST ot SOAP. Wich one should I choose?
There's probably valid arguments for both, depending on what kind of webservices you are communicating with, and what kind of data will be transferred.
While SOAP is a standardized protocol using HTTP to transfer XML data, RESTful web services are not a standard as much as an architectural style.
REST also uses the HTTP protocol for data transfer, but uses the HTTP verbs POST, GET, DELETE, PUT in a stricter sense. Data payloads in RESTful web services can be XML, JSON or other valid MIME types, so you're not limited to XML.