When to use valueChangeListener or f:ajax listener?

What's the difference between the following two pieces of code - with regards to listener placement?

<h:selectOneMenu ...>
    <f:selectItems ... />
    <f:ajax listener="#{bean.listener}" />
</h:selectOneMenu>

and

<h:selectOneMenu ... valueChangeListener="#{bean.listener}">
    <f:selectItems ... />
</h:selectOneMenu>