The main difference is the way the exhaust hoses operate on a single- versus dual-hose configuration. In a single-hose portable AC, warm air is pulled from the room, runs through the AC compressor and refrigerant system to cool down, and the heated air and moisture is exhausted through the hose outside.
Dual-hose portable air conditioners have two separate exhaust hoses. One draws air in from the outside while the second hose expels the heated air and moisture outside. This dual hose system prevents cool air produced by the AC from being exhausted out of the room, a situation that can occur with single-hose portable ACs, making single-hose ACs less efficient at cooling a room.
Single-hose portable air conditioners are lower cost to purchase and still an effective option for smaller rooms with good insulation and fewer warm appliances. For larger rooms and more demanding cooling situations, a dual-hose portable air conditioner may be the preferred choice as they generally cool faster and more efficiently than their single-hose counterparts. Please note that when you buy a dual-hose portable AC, it can easily be modified to be a single-hose unit (by removing one of the hoses) for less-demanding room environments.