Iron and manganese filters look alike from the outside: a tank, a control head, a drain line, and a sales page full of words like 'advanced' and 'high capacity.' What actually separates one unit from another is how it deals with the metal and how it cleans itself. Start there, and most of the other differences fall into place.
What the filter has to do
Dissolved iron and manganese are invisible until they meet air or an oxidizer. Then they turn into solid particles that a filter can trap. A good iron filter has to make that conversion reliably, catch the particles, and release them during cleaning. If any one of those steps is weak, you get staining at the fixtures, or the filter needs cleaning far more often than the brochure suggests.
Ask how the metal gets oxidized
Most residential units use one of two approaches. Some inject air into the water before the media bed. Others add a chemical oxidizer such as chlorine or potassium permanganate, often fed by a separate small tank or a metered pump. Ask the provider which method the unit uses, what happens when the oxidizer runs low, and who refills it. A unit that depends on a consumable oxidizer has a recurring cost and a maintenance task that an air-based unit may not have. Neither approach is automatically better. The right one depends on your water, which is why the provider should be able to explain the choice in terms of your numbers.
Check the cleaning cycle and the water it uses
Iron filters clean themselves by backwashing, and the cleaning cycle uses water and drains it. Ask how often the cycle runs, how long it lasts, and how much water each cycle sends down the drain. Ask whether the cycle timing is adjustable and whether the control head can be set to match your household's use. A unit that backwashes on a fixed schedule regardless of use can waste water in a quiet house and fall behind in a busy one.
Look at the media and its service life
Ask what media the unit uses, who makes it, and what the replacement interval looks like in practice. Media that is only sold by one provider is harder to replace on your own schedule. Ask for the replacement procedure in writing, including whether it is a homeowner task or requires a technician. Compare that against the warranty terms so you know what the provider covers and what you pay for.
Questions to put to each provider
Use the same list for every bid so the answers can be compared side by side.
Which oxidation method does this unit use, and what does it need to keep running?
How often does it backwash, and how much water does each cycle use?
What media does it use, and how is that media replaced?
What does routine service cost you in parts and labor, and who supplies the parts?
What exactly does the warranty cover, and for how long?
Can you show me a unit already running on water like mine?
How to weigh the answers
Write each answer next to the same question for each provider. A provider who can answer every question plainly and in detail is giving you more useful information than one who answers with a brand name and a sales promise. If you want to see how a site like this one weighs its criteria when it assigns grades, the explanation is on the How Grades Work page. Use the same kind of thinking here: decide what matters for your water and your household before you let a presentation decide for you.