Whole-house filtration and UV systems come in only two sizes - based on the length of the catridges: 10" or 20" . The filtration capability of both - is the same . The filtration capability between different branded systems - is the same . The only differences are: a) the quantity of water flow that each cartridge can deal with; b) the servicing cost of the cartridges. Remember: 10" systems: are designed for your average NZ residential home. 20" systems: are designed for commercial properties, irrigation systems, and larger than average residential properties (3+ bathrooms).