A tank type holding more than a cubic foot of carbon is often used for the whole house. A size can be found for any problem the home owner is likely to solve with an activated carbon filter. Obviously, larger units require more space and if space is not available near the point of use, additional plumbing will be required. Faucet- mounted and countertop units are the easiest to install and service when needed. All of the remainder require working with the plumbing system.
Cost is obviously a factor.
You may wish to treat all the water or at least all the water used for drinking, cooking, bathing and laundry, but large units cost more money. Maintenance cost for replacement of the carbon or recharging with hot water is another factor that should be considered. The filter cartridge replacement interval will determine the annual maintenance cost. Finally, the time required to replace the cartridge or filter material is also a cost or at least a convenience factor. You may service the filter yourself, or you may have someone else provide this service.