An air purifier is a device which removes contaminants from the air. These devices are commonly marketed as being beneficial to allergy sufferers, asthmatics, and at reducing or eliminating secondhand tobacco smoke. Commercial grade air purifiers are manufactured as either small stand-alone units or larger units that can be affixed to an air handler unit (AHU) or to an HVAC unit found in the medical, industrial, and commercial industries (Wikipedia Definition)
An electronic air cleaner works by charging particles in the air and then collecting them using an opposite charge, like a magnet. Because small particles are charged, the electronic air cleaner is much more efficient than traditional filters. Very small particles that would normally pass through a filter, such as smoke and dust, are captured along with larger particles.
The 5" filter is way better for the overall operation of your entire HVAC system because it will not restrict airflow the way a 1" filter will do. At the same time, because of how much more surface area there is, a 5" filter will capture more airbourne contaminants due to a slower air flow rate across the filter.