IV/IV withholds 138,00 € more per year than III/V from this household's paychecks, but this is a cash-flow difference in monthly withholding, not a difference in your true annual tax bill. Under Ehegattensplitting, a married couple's actual annual tax liability is the same regardless of which Steuerklasse combination you choose, and any over- or under-withholding is settled when you file your Steuererklärung. Choosing V does lower the net income used as the basis for wage-replacement benefits (Elterngeld, Arbeitslosengeld, Kurzarbeitergeld) for that spouse, which is worth weighing if either of you expects to claim one of those. See the FAQ for more, including the Faktorverfahren as a third option that keeps withholding accurate per spouse.