Find a dealer that sells both Kinetico and RainSoft products. These dealers will be able to give you an expert-level breakdown of the differences between the two companies, and will also be able to tell you their overall customer satisfaction level. If one Kinetico unit sells dramatically more than a RainSoft equivalent (or vice versa), the dealer will be able to tell you exactly why.
Look at the technical specifications breakdowns for each company's products. The websites of both companies include detailed specifications of all the products sold. By having the product website for a Kinetico product on screen at the same time as the RainSoft equivalent, you will be able to compare the two specifications breakdowns side-by-side and quickly see what one unit can do that the other one can't.
Read reviews. Sites like Consumer Reports feature expert reviews and everyday user reviews for products by Kinetico and RainSoft. These experts try out water softener and purification products from the two companies under a variety of conditions and report on their findings. The services will also make a distinction as to which one is easier to use and which one is harder.
Look at the energy efficiency of the water softeners from both companies. For example, many of the models in Kinetico's series of water softeners are powered by the energy of flowing water and use no electricity. RainSoft's water softeners, by association, use electricity to power the motor on the inside of the unit.
Find the products that one of the companies makes that the other one does not. For example, Kinetico only produces water softeners, drinking water systems and whole house specialty drinking systems. RainSoft produces these types of devices in addition to standard air purification systems.