Merge tag support in Mandrill is live now, and you can see the docs for it in the send-template API call now:
So you can use a merge tag of your choosing, and the set it in the API call via either the global_merge_vars or merge_vars variable. The global option will set a dynamic part for all recipients. The latter one is a per-recipient merge var that allows you to specify which recipients get which dynamic replacements.
The template_content struct is a different type of dynamic replacement, and it generally operates at the dom/template level. You specify it like this:
<div mc:edit="my_content_region">Some default content.</div>
Now you can leverage the template_content struct to replace the CONTENTS of that div tag (nothing in the tag itself, just the contents) with anything you want. You'd specify the name as "my_content_region" and the content as whatever you would like to appear inside the tag.
It is up to you which style is easier or more appropriate for you, but generally we think of the mc:edit regions as template-level replacements (perhaps you want to change out large portions of HTML?) and the merge tags are more for copy-level replacements (names, dates, etc).
Let us know if we can be of further assistance.