Vertical and a horizontal chamber each have their own benefits.
Horizontal Chambers: People who prefer to lay down during the session:
Those trying to recover from sports injuries or those that can't sit up right for long times prefer to lay down in the chambers on their back. If the horizontal chamber is large enough there is room to exercise and move about. Any chamber less than 28 inches usually is too small for those that don't want to only lay down.
Some people are sick and need to preserve their energy and it is hard to sit upright for an entire 1.5 to 2 hour session. Many people lay on an infrared mat and raise their core temperature. It is much easier to do this when laying on the infrared mat. Those recovering from injuries usually prefer the infrared mat. Also, it is easier to breathe for most people when laying on their back. The horizontal chamber is the most popular for non EWOT reasons.
Vertical Chambers: People who prefer to sit up during the session
The vertical chamber is the type that is easiest for getting in an out if someone has mobility issues. There is also a wheelchair accessible version that will let a wheelchair roll right in the chamber. Their are 2 main sizes, the Vertical Dive and the Grand Vertical Dive. The smaller one is large enough for sitting room and just a little movement. The grand size is what is popular with athletes doing EWOT and off season training. Working out in a large vertical chamber makes a big difference in my workout. I use my oximeter and once my blood is at the point I want it at, that is when I begin my workout. I strongly recommend getting a pulse oximeter. It can save someone a lot of time and rid the confusion on how long to stay in the chamber. See my Vertical Chamber page.