With cleaner indoor air becoming not only demanded by building occupants and required by increasing regulation it also is starting to have documented studies showing productivity gains by implementing programs to ensure clean indoor air - even in typically dusty/dirty manufacturing, warehousing, and other industrial facilities. These revelations have lead to the explosion of industrial grade HEPA air purifiers - each with unique differences that make them optimal for different applications.
The biggest difference between your options is the size of the space and the application, industrial air filtration requires much larger and higher capacity air cleaners than one that will be used for office space. This means to do an entire facility there may be a mix of styles as the large production floor/warehouse would need an industrial air filtration system, and the administrative/office area would need units that are smaller and as quiet as possible.
Horizontal HEPA Air Purifier: For Industrial Air Filtration, the larger horizontal style are typically the best bet. They do not require any additional outside air brought in, and they circulate all of the air in the sapce - ensuring that the air is treated without putting additional stress on any existing HVAC system. This makes them a great retrofit option to boost indoor air quality in an industrial facility at minimal cost. An important thing to remember with these is unit placement and airflow pattern is important - if there are too few or poorly arranged horizontal air purifiers they will not operate as an effective system and the results will be less than desired. Becasue so many contractors have rushed to install them without proper sizing and placement these units have gotten kind of a bad name over the years - however when done correctly the results are amazing. (And at a fraction of the initial, ongoing, and operating costs of other industrial air filtration systems)
Office Air Purifier: These units are smaller units with quiet high efficiency fans that allow them to produce good air circulation without being a distraction. Also common in these units is some kind of carbon filtration so that not only ultra-fine particles are filters out, but also molecular pollutants like Nitrogen Oxide (NOx) Ozone (O3) and other VOCs. A "whisper" is typically defined at 30 decibels, so finding one of these units that operates below that noise level will keep it from being an irritant to the people in the office. Also, watch the wattage that the unit will pull as that will give you an idea of the amount of energy it will consume throughout the year - this is not very well monitored/regulated so there are vergy large differences between manufacturers. A good example of a low-noise low-energy HEPA air purifier is the CityM by Camfil which produces only 22 decibels and consumes only 7 watts at it's recommended setting, this equals less than $10 in elecitrical consumption per year if left on 24x7!