When two installers quote the same softener at very different prices, the first question to ask is not which company is cheaper. It is what water each of them assumed. A softener is sized against the hardness of your water and how much of it your household uses. If the two quotes start from different assumptions, the totals cannot be compared fairly.
Find the hardness figure on each quote
Hardness is usually expressed in grains per gallon. A good quote states the figure it used, where that number came from, and how the unit size was calculated from it. If a quote does not show the hardness input, ask for it in writing before you accept any price.
Ask where the number came from
A test result from your own water, taken from a tap you use, is more reliable than an estimate based on the water supplier's general report or a guess. Ask whether the installer tested your water, which tap was tested, and whether the result was recorded on the quote. A quote built on a test is easier to defend than one built on an assumption.
Check whether the unit size matches the input
Once you have the hardness figure, check whether the unit size on each quote fits it. A unit sized too small for your water and household will regenerate more often and may run out of softened water during peak use. A larger unit costs more up front, so an oversized quote is not automatically the better offer. The right comparison is two quotes that use the same hardness input and the same household usage assumption, then differ only in equipment and service.
Note the other inputs that change the price
Household size, daily water use, and whether you also need iron or other treatment all change what the right unit costs. If a quote adds equipment for a water problem you have not had tested, ask why it is there. A separate test result may explain the add-on, or it may show it is unnecessary.
Compare on the same page
Put the quotes next to each other and write the hardness input, the household assumption, and the unit size at the top of each. If those three match, the price difference is real and you can weigh it against service and installation terms. If they do not match, ask each installer to requote on the same numbers before you decide.