For anyone else who needs this, here is an answer:
<xen:if is="{$contentTemplate} == 'forum_list' OR {$contentTemplate} == 'resource_description'">
Maybe it was doable with an array too, like this syntax with the correct template names:
<xen:if is="!in_array({$contentTemplate}, array('message_page', 'error', 'search_form', ......