I'd keep the 3.9% and invest, but with one condition: you have to actually invest it, every month, without touching it. Most people who make the mathematical argument end up with a mortgage and a slightly bigger car. If you know you'll keep the discipline for two years, the numbers are on your side. If you don't, the mortgage is a forced savings plan with a decent return.
Cal
@coopvent_cal
Rebuilt a coop for ventilation and has not had a frostbitten comb since.
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The honest difference is service intervals and how much iron gets through. A cartridge system at 2.1 ppm and 150 gallons a day will need new cartridges every three to six weeks, which is both a real cost and a chore you will resent by August. The two-stage system handles the iron in a backwashing filter so the softener only has to deal with hardness, which is what softeners are actually good at. If 3,400 is out of reach, the middle path I'd take at 1,500 is a single air injection iron filter now and live with 14 grains of hardness until you can add a softener, rather than trying to make one softener do both jobs.
Important one. Routing backwash to a dry well or a separate drain rather than the septic is common practice and worth pricing up front, not after.
Third person recommending the fake paycheck. It's the highest ratio of benefit to effort of anything I did in year one.