This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
XoH as it is more popularly known, is semantically meant to achieve the same objectives as SOAP. So, wherever you intend to use XoH, you can potentially used SOAP.However, the reverse might not necessarily be true.
The choice of XoH over Soap primarily grinds down to a few basic thumb rules(in my opinion)
Will you ever have the need to define the data/service contract more formally defined ? - If you need a more formal definition, Soap is your answer.
Do you want the service to be used inside your organization or across organizations ? - This closely relates to the previous one, a service used across organizations must have a more formal way of describing it
Do you want tooling to generate client stubs - If yes, XoH might restrict you here.
Do you want the vertical and horizontal flexibility offered by SOAP
I am sure there will be more..but these are the only ones which come to my mind immiediately.