which is more efficient to do:
1.
<option value="1"<?php if($th_order==1){echo " SELECTED"; }
?>>Something1</option>
<option value="2"<?php if($th_order==2){echo " SELECTED"; }
?>>Something2</option>
(or)
2.
instead of having the <?php and ?> mixed in the HTML is it better to
echo/print the whole lines?
Really just a matter of personal preference. If I have a mostly PHP page, I would use echo to print out chunks of HTML. If I have a mostly HTML page (or section of a page), I would use separate <?php ?> sections within it. I doubt you'd notice any performance difference between the two so use whichever is easier for you to code.
Larry