Most displays are in landscape orientation, this is the normal 16:9 aspect ratio of television. This works great for many digital signage applications when showing video, when the displays must be oriented in landscape, or the content otherwise lends itself to a wide aspect ratio.
However, don’t rule out portrait, 9:16 aspect ratio. The cool thing about mounting a display in portrait orientation is that is looks more like a poster. You can do some very interesting things with the visual design and in many cases help the user absorb your information faster. A portrait orientation is great for displaying lists, menus, and directory information.