Hi Nadeem,
You have the choice to either create a new method in an extended schema of xtk:enum or in xtk:enumValues (by adding method code in the Administration>Settings>Javascript codes node), or directly code in your client Soap call the xtk.session.Write or WriteCollection (or the entity save method, depending on your way to code).
Creating his own method is a recommended choice to hide the underlying details, for example if you don't want to expose some business rules to the client (and the developer in charge of the client). So the xtk.session.Write is coded in the method code, business rules checks etc.
If the client code is managed by the same developer profile (that has already access to the Adobe Campaign server or client as operator), it is more or less the same, and the main advantage is to avoid any specific documentation 
My recommendation would be the method extension of schema if you need to implement some business rules.
It is very similar to the choice of stored procedures or directly code SQL queries in a client code...
Regards
J-Serge