I am very confused with
Soap curl request
I found here an example
SOAP request in PHP with CURL
and Soap request using SoapClient
PHP
You are right that they fill the same purpose. Though, the SoapClient has a lot of functionality for supporting SOAP requests built in and is using Curl somewhere as well.
As you can see in the discussion on SOAP with Curl the programmer constructs the SOAP envelope, the SoapClient can do that for you. The same holds for doing the method calls.
In the end, it is much easier to use the SoapClient.