Horizontal: Will reduce your gameplay distance around half meter.
Under some distances is slighty apreciated. On a 27" inch I would go for Vertical Set up, You should play from 2,5 or 3 meters wich should not be a pain.
Vertical: It is simplest way to hook it. Also is the best way to know that you are inside a "playable" area, because the Bars are ON THE EDGES.
Enjoy your Topgun