It's the difference in the client library being used.
ExacttargetSoapClient
is a pure PHP library, and mostly outdated.
ET_Client
is the Fuel Client Library (currently labeled SDK), and it utilizes the newer app model and authentication flow using clientId
and clientSecret
. It stream lines much of the older library with better methods.
The Fuel Client library also abstracts our SOAP and REST APIs. Our SOAP API is our oldest and most comprehensive API, but its functionality is limited to the ExactTarget email application, and, like all SOAP APIs, it is fairly heavyweight. Our REST API is newer and less comprehensive, but it exposes a broader set of Marketing Cloud capabilities and is much more lightweight and easy to use (and is getting more comprehensive with every release).
You can get the PHP Client Library (SDK) here - https://github.com/ExactTarget/FuelSDK-PHP/tree/Dot9/