WSDL to use to authenticate the other service and will get Session Id in response. Which will they use to make other callouts in Salesforce.
You can also use enterprise WSDL here
The primary differences between the two WSDL that Salesforce provide are
a) The Enterprise WSDL is strongly typed.
b) The Enterprise WSDL is tied (bound) to a specific configuration of Salesforce (ie. a specific organization's Salesforce configuration).
c) The Enterprise WSDL changes if modifications (e.g custom fields or custom objects) are made to an organization's Salesforce configuration.
For the reasons outlined above, the
Enterprise WSDL is intended primarily for Customers.
a) The Partner WSDL is loosely typed.
b) The Partner WSDL can be used to reflect against/interrogate any configuration of Salesforce (ie. any organization's Salesforce configuration).
c) The Partner WSDL is static, and hence does not change if modifications are made to an organization's Salesforce configuration.