If you want eye-searing brightness, go with the 27%, but you won't get the benefits outlined below.
My preference is the 13%. But you'll need to crank up your image brightness.
Adjust the contrast way up and then adjust the brightness so that the black background totally disappears.
With the 13%. you shouldn't need a bezel, and vertically oriented games won't look like they are being played on a horizontally oriented monitor.
RandyT