Template Files - PHP or HTML
Overhead, if there is any, is of little concern. This is pretty much a stylistic concern. As such, the important part is a consensus among developers so there is clear readability expectations.

drupal.org has documentation when it comes to template coding standards. In this case, little-as-possible PHP tags usage is preferred (the concern here is readable HTML rendering, not code execution) with use of PHP's alternative control structure:

&lt;?php if ($a == $b):?&gt;
   a equals b
&lt;?php else: ?&gt;
   a does not equal b
&lt;?php endif; ?&gt;


See the documentation for further style guidelines with templates.