Think about what will happen when precipitation lands on a horizontal roof as compared to a vertical roof, and you will understand the tradeoff that you’ll be making with this purchase. A vertical carport does a great job of shedding rain and snow, since the roof panels are oriented in such a way that it can just flow right off without any trouble. In a horizontal orientation, that precipitation is going to need to move off the front or the back, instead of the sides.
That’s a less-efficient option, and it means that your carport may not be able to get rid of rain and snow quite so easily. With that said, some buyers are not going to need to concern themselves with that issue. For instance, if you live in a dry environment where heavy rain or snow is uncommon, the horizontal layout might work perfectly for your needs. Or, if the carport is going to be set partially under some trees where it will have a degree of protection from rain and snow, the horizontal pattern may work beautifully.