Realistically, there is no meaningful difference between horizontal and vertical air compressors except for the way that the hardware is arranged. The basic idea is that a it will help save floor space since the length of its tanks runs up and down as opposed to across. Typically the motor will sit at the base of the unit with the tank running up above it. As a consequence, when left sitting in a given location, it takes up less horizontal space, though of course it takes up more vertical space.
The primary advantage of vertical air compressors is to small businesses and offices that need compressed air but have limited floors pace and would prefer them be stored out of sight. A good example might be a dentist’s office (as they use a lot of pneumatic tools in dental care) or a small rented workshop. Most vertical compressors can be wheeled about in a fashion similar to moving a hand truck and can therefore easily be positioned in a disused closet or corner. This portability also makes these more popular if the compressor needs to be moved about frequently.
Otherwise beyond the question of horizontal space saved, there is no other meaningful difference between a horizontal and vertical air compressor. Air compressors of all kinds can be found in both vertical and horizontal styles and the direction of the tank has no bearing whatsoever on the general performance of the compressor. Therefore, if space is not a significant issue, then there is no need to focus specifically on the direction that the air compressor’s tank lays. In fact, if you intend to hide the compressor under a work table or cabinet, then buying a vertical compressor would in fact be a mistake.
Obviously this is only a relevant concern for air compressors that are large enough to justify the concern, specifically those with larger tanks. When dealing with compressors with very small tanks (two gallon, six gallon, eight gallon, etc.) it does not really make any difference which way the tanks are turned with respect to horizontal floor space. This only becomes a real issue when dealing with tanks of about twelve gallons or higher. Since tanks play an important role in maintaining an even flow of air and the like which is required for sensitive work (like dentistry), having a larger tank is typically and important thing.
Since so many of the applications where floor space is an active concern also coincide with locations where volume is an issue, many of the companies that specialize in the production of quiet air compressors offer their products in a vertical formation. For example, busy offices or shared workspaces are places where both floor space and noise are joint concerns. Since air compression has never really been a silent technology until recently, there are many small producers that have opted to specialize in producing silent compressors. These specialized air compressors are also frequently vertical, simply because this compliments many of the more common applications.