From a purist perspective, I would like to say that the format of an image is defined by the image, that the subject matter will determine whether an image should be vertical or horizonatal or square or some free form crop. It is a bit unrealistic to take this position, however.
I knew a photography instructor years ago who stated in his classes that Horizontal was the ideal format. He felt that an image that was horizontal was “at rest.” It makes sense given that a horizontal image has a larger base. Yet, this same instructor was constantly creating images that were vertical format. The reason is quite simple. The best paying sale of an image is a cover. Magazines tend to be vertical in format. So, if you want the cover you had betteer be sending them vertical images. In fact, for many years magazines would only do partial page prints of horizontal images. Thus, the pay for having a horizontal image published one was greatly reduced. Commercially, it has long been accepted, a vertical format is the ideal.
Now days magazines will often include full page prints of horizontal images. The reader simply has to rotate the magazine to view the image. This increases the relative value of a horizontal format but it still can not occupy the cover.
I wonder what the future holds. Magazine and print book sales are down. E-zines and E-books are on the rise. More and more people are turning to their computer, pad, kindle or even phone to do thier reading. Web publishing is taking over and it will change the photography industry.
The format of a monitor is horizontal. When I am reveiwing my images I find I am often much less impacted by the vertical images for no other reason than the size. A vertical image can not display as large on the screen. Size does matter. I can overcome this prejedice mentally but the emotional impact just can not be obtained. This will be true for our viewers as well.
The future ideal format will be horizontal. I have no doubt on this. I will still create my images based on content and, when I have vertical content, I will create vertical images. But I believe it will be the horizontal image that will pay the bills.