You can choose what's used (verbwise, GET/POST etc) as well as the response format (JSON/XML etc) with ScriptMethod.
Here's more:
ScriptMethod on MSDN[
^]
WebMethod was traditionally used with web services so that clients would be able to call them via script. I have used it to return simple values and XML. JSON is a different beast, a gentler, kinder, lightweight beast that is fun to snuggle with. See
here[
^] for details.
Cheers.