As well as deciding which of the water softener types the best for you, you'll also have to decide what form of salt you want to use. You'll find some have benefits above and beyond price and efficiency.
This type isn't usually used for water softeners, although some factors are persuading more people to use potassium. It can be almost twice as expensive, is harder to find and works less efficiently (so more is required), but it is more environmentally friendly, eliminates water softener health concerns associated with sodium chloride and isn't detrimental to plant life - in fact, it's a nutrient.
This mineral is the standard for most ion exchange water softeners. It's a lot cheaper than other types and therefore much more popular, but several concerns have been voiced. Some homeowners stay away from salt-using water conditioners altogether, citing environmental concerns over salt waste, and some choose water softener brands that use sodium chloride more efficiently.
Salt efficiency and cost
The less total solubility of water softener salts, the more frequently you'll have to clean the water reservoir. Often with the more soluble salts, price goes up, so if you want easy cleaning regardless of the price, look to evaporated salt. For you cost-conscious folks who don't mind a little hard work, the best deal is in rock salt - but if water usage is high, you might want to consider the slightly more soluble solar salt. Some water softeners will only function well with a certain product, so check to make sure it will accommodate your salt of choice.
Mixing salt types in the water softener isn't recommended, although as soon as one type of salt has emptied out, it will likely be safe to add a different kind. Alternating products can keep costs down and help keep cleaning to a minimum.
Salt is being used every time the softener regenerates. The more water you use, the quicker you'll go through salt. A good idea is to check the softener once a month and keep the salt at at least half-full all the time. If you let it go completely empty, it will take a bit of time to get back to softening again. If it immediately starts regenerating when you add new salt, it might not be running correctly.