As with all things in life - it is a compromise, a puller TR gets clear airflow which is good but blows out against the tail boom/pylon which is bad (like standing on your own shoelaces and jumping) a pusher TR gets disturbed air around the tail boom/pylon but blows into clear air which is good. I am sure that St Nick (Lappos that is) has a more detailed explanation and comparison of the 2 but suffice to say that pushers outnumber pullers by quite a margin.