HEPA (High Efficiency Particulate Air): This is the most efficient type of filter and can remove the smallest particles with the greatest efficiency. True HEPA is 99.97% efficient, so you want to look for a model with True HEPA (especially if you have allergies).
ION GENERATOR or IONIZER: This filter generates negatively charged ions. They act as a magnet for dust and dirt particles that are removed from the air by adhering to the filter or other objects in the room.
HYBRID: Utilizes more than one filter which work together in the unit to capture different sized particles from the air.
When choosing a unit, look closely at the AIR CHANGE/EFFICIENCY RATE. This is the number of times that the unit can filter out the air in a specified sized room each hour. When making your comparisons, remember that the more changes per hour in the same size room will determine the most efficient machine. The optimal air change rate is 6 times per hour. You should be purchasing an air purifier that can accommodate the room size you want it to filter. For example, if your room dimensions are 12' x 15', then look for the model which will provide the highest air change rate for a room close to that size, and preferably one that is True HEPA.
Obviously, the other factor is the size and style of the unit. This is determined by the size of the room in which it will be used. Some units have airflow from only one side, while others can have a 360-degree airflow. This will determine where you can place the unit in your room. Almost all units come in neutral colors such as white, gray or beige, to blend into the room. The focus here really is function, not style, so choose the unit that will be the most effective in each room.