Two written estimates can have totals that look close while one covers far less work. The number on the first page tells you very little. The gap between two quotes is usually hidden in what each one leaves out, so compare the line items before you compare the totals.
Start with the equipment line
Check the exact model named on each quote, not just the brand. Confirm the stated capacity and whether it matches the sizing you have been given. Ask whether the control type is the same on both quotes. A quote that names a different control or a smaller tank is not the same product, even if the company name looks familiar.
Check the work that is not equipment
Look for these lines on each estimate. Removal and disposal of the old unit. A bypass valve, and where it will be installed. The drain connection, including whether a new drain line or fitting is needed. An electrical outlet, if one is required near the unit. Any permit or inspection the company expects to handle, or that it says you must handle yourself. Startup, including the first regeneration and a test of the water at the tap. If a line is missing, ask whether it is included or an extra charge. Silence on a line is not the same as no charge.
Ask what happens after the install
Ask how service calls are handled, who responds, and whether the company stands behind the work it installs. Ask which parts are covered by the manufacturer and which work is covered by the installer, and get the answer in writing. Do not assume the two are the same.
Use the rating method when you compare
If you have read a rating page for a company, check how its scores were built before you lean on them. The rating methodology explains the criteria and weights, and it helps you see what a score actually measures. Use the scores to narrow your list, then compare the written estimates on the points above.
Put the two quotes side by side
Set the quotes in matching rows: equipment model, capacity, control type, old unit removal, bypass, drain, outlet, permit, startup, and post-install service. Any blank row is a question to send back to the seller. The cheaper quote is only cheaper if its rows match the more expensive one.