A quote is only a total until you know what it includes. Two providers can both say they will install a treatment system and still be offering different equipment, different work, and different promises. The fastest way to compare them is to build one scope sheet and send the same description to every provider.
Write the scope you are pricing
Start with the problem you are treating and the test result behind it. Write down the flow you need, the stages the water calls for, and the place where the system will be installed. Send that same description to each provider. If one provider quotes a different scope, you are comparing two different jobs.
Line up the equipment rows
For each quote, record the brand and model of each stage, the rated flow, the media or cartridge type, the certification listing if any, and the tank size. Mark any row that is missing. A missing row is a question to ask, not a reason to assume the lowest total wins.
Line up the work rows
Installation quotes often leave out the same work: fittings, drain connection, electrical needs, permit handling, pipe removal, and cleanup. Mark each as included, excluded, or the customer's responsibility. Compare the included rows across quotes, and treat any excluded row as an added cost you still have to price.
Line up the service promises
Write down what each provider says about the warranty, the first service visit, the replacement schedule for media, and who you call after the install. Ask whether those promises appear in writing. A verbal promise cannot be checked later, so it should not be scored as if it were a term of the contract.
Check claims against the source
When a provider cites a certification, ask for the listing number and look it up with the certifying body yourself. Compare the model number on the listing to the model in the quote. A mismatch is a common reason a claim does not hold up.
Score the sheet, then compare totals
Once every row is filled in, compare the totals only after you have checked which rows differ between quotes. The methodology page explains how this site matches systems and compares providers, which can help you set up the same checklist before you request quotes.