What counts is how it handles the nasties that only show up under an electron microscope. Short of taking the whole lot to a public health laboratory, I've worked on the basis of supplied technical data, and judged the different models on their ease of use, and the palatability of the filtered or purified water. In fact, all filter out particulate matter, cysts and bacteria - those with added iodine provide the only sure treatment for viruses. Filters are either membrane or surface types, which quickly get coated in gunge but are easy to clean, or depth filters, made of thick porous materials like activated carbon or unglazed ceramic. The latter rely on progressive filtration, and so contaminants move further into the material.
Some depth filters are partially cleanable, either by backwashing, or by brushing the outer surface.
An additional element of treatment which doses the water with iodine to kill off bacteria and viruses. In areas where viruses are likely to be prevalent - notably third world countries with poor sanitation, this kind of unit provides the best peace of mind. Not everyone likes the taste of iodine, however, and prolonged use beyond three months is not recommended. Activated carbon filters will remove the taste of iodine. The Pur filters can be fitted with an optional carbon cartridge attachment which does this.