I think you meant horizontal instead of vertical, which would be correct if you just try to balnce it so as to get the nose and tail level. If a plane is nose heavy ( nose down in your picture) then it will be more stable, track straighter, and easier to control for the most part, but it will land at a little faster speed and need a little room. Even a plane that is too nose heavy can still be flown successfully, it just wont be very gracefull. A slightly tail heavy plane will land slower and be more agile, but could be harder to control, especially on a windy day. A plane that is too tail heavy will most likely be too hard to control and is likely to crash.