The air purifier you choose will depend on the size of your room. The bigger the room the more powerful the purifier needs to be. There are two measures, CADR-clean air delivery rate which measures the amount of air produced. So a bigger room will require a higher CADR value. This number should be provided by the company that makes the air purifier.
Another good measure to use is how many times the air purifier can change the air in one hour. More frequent air changes is best to maintain good air quality. 3x p/hr air exchange is good, best practise is around 5x p/hr air exchange per hour. Most companies will advise how many times the purifier will change the air in a room based on its size. 

There are two different types of technology to choose from: HEPA filters and electrostatic filters. While both filter particles, viruses, pollen and other pollutants from the air, the difference between them lies in the way they are maintained. The HEPA filter needs to be replaced regularly. You will need to clean the electrostatic filter regularly also. Roger uses a Dual Filter with HEPA H12 & Activated Carbon. They should be replaced every 8-12mths.  

Only activated carbon filters can remove unpleasant odours and gases from the air. HEPA filters collect all the airborne particles, whilst activated carbon filters remove all the unseen gases and smells within the home. Roger air purifiers have a Dual Filter so you can achieve clean air that smells fresh with one device & one filter. Roger air purifiers are sold with a H12 HEPA Dual Filter. (H12 = 99.5% efficiency).

It is worth looking at the number of decibels the appliance will produce. If you are using your air purifier in a living room, study or bedroom this is an important consideration. Air Purifiers should be left on all the time as its the best way to purify the air effectively. It would be beneficial to ensure your purifier operates quietly on a low setting so that it will not disturb you.