Most of the OA teams of Chief Delphi have converged on the same shooter, which is top and bottom rollers with a gap in the middle. It generally performs the best, because it deforms the game pieces less, as they are not laterally deformed by middle rollers.
Choice on rollers has been split between a couple options (Colsons, Stealth wheels, etc), just keep in mind that the larger diameter wheels (3in/4in) are going to shoot the game piece a lot farther then smaller 2 in wheels, because of the much larger radii.