Would a good score (god willing) outweigh putting it as 2nd choice should the comp be oversubscribed in catchment this coming year?
IT is not a question of one out weighing the other. In the scenario you suggest, IF the score was above the qualifying mark, your DD would get offered Tiffin. I re-iterate the points others have made, your dd will get the highest ranked school for which she qualifies
, if she does not qualify
for the comprehensive due to a combination of geography and a high number of closer applicants this year, then she gets Tiffin, assuming she has a sufficiently high score. IF, however, she scores sufficiently highly but you have ranked the comprehensive above Tiffin, then you will still get the comprehensive unless
due to the geographical factors you mention she does not qualify.