If you are looking to compare water treatment systems, each type's positives and negatives need to be taken into account. I'll outline the types of water treatment systems which are suitable for home use only.
Firstly, there is the stand alone filter that sits on a bench. These stone, steel or plastic vessels are filled by hand from water from the tap or tank. The water is filtered as it passes through ceramic and/or carbon filters via gravity. Just the thing for your daily drinking water. They can be quite attractive but you do need a 'spot' on your bench for them and the better ones aren't very cheap.
For home use, distillation is limited to drinking water only. Water is heated and the resulting steam is collected in a vessel. Unfortunately, not all of the water's chemicals are removed as those with a lower boiling point than water, pass into the collection chamber. Again, taste suffers because of the removal of those essential trace elements. Best left to industry, and for filling your car battery, where demineralized water is needed.
Next, are the types of filters that are attached to the mains water supply. These can be installed on the sink, under the sink, in the shower, or at the point of entry to the house.
Most people would only consider water treatment systems that purified drinking water only. Because of the nature of contaminants found in water, in particular chlorine, a shower filter and/or even a whole-house may be considered necessary for your needs. Chlorine gets into your skin through open pores while showering. The fumes from the shower and appliances such as the washing machine and dish washer, when using untreated water, can cause real problems, especially for asthmatics.
The pressure of the water mains forces water through the filtration system. These are the most commonly used because of their cost and convenience. To compare water treatment systems which use your mains water supply, we need to look at how they operate and the effectiveness of each.
Water is forced through or a semi permeable membrane, removing many toxins in the process. They are quite expensive to operate (filter replacement) and slow the water flow down considerably, but their biggest negative is that, along with impurities, they also take out beneficial minerals such as calcium and magnesium which are very important to proper body functioning and for better tasting water.
Water passes through or around an Ultra Violet tube, effectively killing bacteria and viruses. If your only concern is the bugs in the water, these are an excellent choice. They should be combined with other types of filters, if the other contaminants found in water are to be handled.
An electric current is passed through water to raise its PH, ie increase its alkalinity. Some swear by them, others say they can't work in a water environment and are an expensive substitute for a diet containing calcium. I'm staying out of the argument for the moment.
These have been tried and tested over many many years. When water is forced through a carbon material, such as charcoal or coconut shell, chemicals and most other toxins collect on its surface--called adsorption. The combination of chemical bonding and mechanical straining makes a carbon filter, especially if it is a multi-stage process, the best of the water treatment systems. Also, they allow essential minerals like calcium and magnesium to pass.
Because they can be produced inexpensively and their replacement filters are reasonably priced, to compare water treatment systems without them having a carbon filter as part of their construction doesn't make a lot of sense. See below for excellent quality water filters selling at a discount.