BTU rating
PACs are rated in BTU. The greater the BTU of a unit, the more cooling power it has.

0-150 square feet=7500 BTU
150-250 square feet=9000-10,000 BTU
250-450 square feet=12000-14,000 BTU

These sizing guidelines can sometimes differ from the maximum suggested room size given by the manufacturer, and high heat loads can impair the effectiveness of any air conditioner. Keep in mind that a PAC with too little cooling power will not cool your space adequately, while a PAC with too much power will short cycle, which will burn out the compressor, and not dehumidify your space well at all.

Condensate disposal methods
All PACs dehumidify the air that passes through them. The water removed from the air is called condensate. This condensate is collected in a bucket inside the PAC, but different models dispose of it in different ways:

Drip: These PACs have removable buckets that fill up relatively quickly (every 2-8 hours, depending on conditions) and must be emptied by hand.
Partial Drip: These PACs evaporate most of the condensate and vent it out the exhaust hose. The remainder must be manually emptied from their non-removable internal bucket about once every 12 hours-several days. A few PACs use an internal bucket and drain pump to pump the condensate out via a small tube that fits inside the exhaust hose.
No Drip: Some PACs exhaust all of the water into the air. This feature is nice because it allows you to run the unit without having to worry about water accumulation that must be emptied from time to time.

One hose vs two hoses
Some PACs feature a dual-hose design. This design cools the compressor of the PAC with air from outside your room. This increases the efficiency of the PAC, and also avoids creating a negative air pressure in the room (as single hose PACs exhaust air from your room, causing the room to suck in air from outside). Two potential downsides to the dual tube design exist: if the air outside is too hot (>95ºF) or too humid (>80%) you will not be able to use the second hose, and the dual tube design can be a bit more cumbersome to deal with if you are planning on moving your PAC around frequently.

Modes
Of course, all PACs function as air conditioners, and they can all also be used as fans, but some PACs can be used as dehumidifiers (independent of the air conditioning function) and/or heaters. It may be worth the extra money to get a unit that offers enough different modes to be used year-round.

Control style

Some PACs have electronic controls while others have manual controls. Models with electronic controls generally offer more amenities, such as remote controls, louvers that can be set to auto-swing, and more advanced programmable features. However, one large downside of some models with electronic controls is that if the power to the unit is interrupted while the unit is on, the unit will default to "off" even after the power has been restored. PACs with manual controls tend to be simpler to operate, and they can restart cooling after a power outage without your intervention.

Venting kit
Different PACs have different types of window kits. All window kits share the same basic goal: they are a "filler kit" designed to allow you to vent the exhaust hose out of a window without hot air from outside rushing in. The window kits differ in their material of construction and their maximum widths. Additionally, a ceiling venting kit is available for select models.

NOTE: ALL PACS MUST BE EXHAUSTED SOMEHOW. This can be done a variety of ways, but you must get the hot air out of the room somehow into empty space where it can dissipate. Exhaust hoses should also not be extended longer than 10 feet as they will radiate heat.