Your hands will become shaky if there aren't already
The position with camera in portrait orientation is less stable since you have have only 2 fixed points for the camera (left arm and head) but the right arm is up.
In landscape orientation both arms are close to the body and you have more stability. But some people also have chicken wings in that way ... :bsmilie:
If you are holding your vertically camera for short duration there is not much different, but if you are taking mostly portrait shots with a 200mm lens for a period of 30mins to 1hours.... than a vertical grip do come in handy...