What to consider before buying a portable air conditioner.
The most important thing is that you want to get the proper power rating for the area being cooled. Pay close attention to the floor plan and insulation in order to get the right number of BTUs.
After deciding on your portable air conditioner BTU rating, much of the decision comes down to your own preference. The number of exhaust hoses or if you want oscillating or fixed louvers are purely a matter of choice. Deciding what portable air conditioner you want depends on several factors, including; the space being cooled, how long you intend to keep the portable air conditioner on at one time and whether there are dedicated personnel who are allowed to adjust it as needed. If noise levels are an issue, become familiar with the DBA rating that is listed in the technical specifications of each air conditioner. 

The proper BTU ratings for areas being covered.
Portable air conditioners are rated in British Thermal Units (BTU). The greater the BTU, the more cooling power the portable air conditioner has. The following are general guidelines for the maximum room size to BTUs that are typically recommended for portable air conditioners:

• 7,500 BTU: 150 square feet
• 9,000 BTU: 250 square feet
  
• 10,000 BTU: 300 square feet
• 11,000 BTU: 350 square feet
• 12,000 BTU: 400 square feet
• 13,000 BTU: 430 square feet
• 14,000 BTU: 500 square feet
  

Other things to consider when choosing a portable air conditioner
There are many different brands and levels of portable air conditioners available to consumers these days. Many of these are evaporative, which means there are no water tanks to empty. Still others require you to drain the internal water tank every 12 hours to several days. The type of portable air conditioner you choose may depend greatly on where you intend to exhaust it. For instance, if you are going to exhaust the air conditioner through the attic or a drop ceiling (for units that qualify for attic ventilation), you do not want moisture to be exhausted with the hot air so you would not use the evaporative type. However, if you are exhausting the unit through a window, the evaporative type would be a good choice.

You might also want to consider the portable air conditioner's other modes of operation you will require. Portable air conditioners dehumidify the air and can also be used as fans. Some units can be used as dehumidifiers (separately from the air conditioning function) and/or heaters.

Another consideration is picking the control type of portable air conditioner that fits your needs. You can choose electronic controls, which offer features such as remote control, louvers that can be set to swing automatically or advanced programmable timers. You may also choose the manual control type, which is simpler to operate and can restart cooling process after a power outage without human intervention.

Finally, the systems energy efficiency ratio (EER) rating and sound level will play an important part in your decision to choose a specific portable air conditioner as well. The higher the EER rating, the more efficient the unit is.