If you are going to get a portable one, make sure it is a dual hose type.
Think of it this way:
Single hose: exhaust ONLY... So unit draws air from the room, a portion of that drawn air is cooled and sent back into the room. Another portion of that drawn air has heat "dumped" into it, then it goes into the exhaust hose and is sent outside. What you are effectively doing is one good thing, two bad things. Good=the room gets somewhat cooler. Bad=first, that exhausted air (drawn from the room) has to come from somewhere, so you are drawing warm air from somewhere else in your house into the room you are trying to cool. This air has to come from somewhere... if your house is leaking it comes from outside via cracks, if your house is tight it will get pulled in through combustion air sources (pipes means to supply combustion air to furnaces, gas fireplaces, hot water tanks etc. Second, you are also pulling some of your "just-cooled" air into the air intake... a portion is re-cooled, but a portion also exhausted with heat dumped into it. Think of it as an inflow into the unit, being split into two exhaust flows... one is cooled and sent back into the room, the other gets heat dumped into it and is sent outside. Because the cool exhaust is less than the overall intake, the unit is always pulling more air into it than is actually coming back out the cold air vents into the room.
This is why single hose portable air conditioners are extremely inefficient and costly to run. Because of the exhaust side (hot) only being outgoing from the house, and there effectively being this melding of the hot and cold sides of the unit.
Dual hose: Unit pulls air from the room, cools it, and dumps it back into the room. The hot side pulls already warm air from outside through one hose, dumps heat into it, and then exhausts it. Thus the hot/cold sides of the unit are separate. Far more efficient, because you are not pulling air from inside the house to exhaust, nor are you exhausting your just-cooled air. The dual hose unit effectively splits the hot and cold sides completely.
You can think of a dual hose unit as a window unit, with the unit moved into the room, but the hotside still connected to the outside via two hoses.
These units are more expensive to buy, but significantly cheaper to run and more effective at cooling a room/area.
GB