Without knowing how your filtering already makes it a little difficult but obviously you don't want to give away the rules. To decide between the two selections maybe use a filter thats not readily available or considered by the public. i.e. don't use a filter thats on all TAB screens around the country that everyone takes into considering (placed last start, top barrier etc etc).

Maybe go with the horse that improved the most from Turn to finish last start (i.e. was 8th at turn and finished 3rd) or possible look at the sex of the horses and see if a particular sex has won more for your system? Maybe only back those that have an apprentice on, or haven't had a change of jockey?

I'm just throwing random ideas here but look outside the square a bit and you might find that it tips you from that break even position to a profitable position.