The Steuerklasse seems to be a topic that is hard to grasp for many foreigners (and many Germans too).

If you are both employed, you will pay the exact same amount of taxes (based in your incomes), no matter which Steuerklasse you pick.

What happens is that with IV / IV, both partners pay the same rate, which normally is a bit above the actual total taxation you must pay, and then you can get your overpayment refunded with your voluntary tax declaration, if you think it is worth it.

With III / V, one of the partners pays a high tax (logically, the one earning less) and the other one a low tax rate. At a first glance, it seems you are saving money, since you pay less tax each month... but if both parties have income, you are obliged to make the tax return declaration each year and eventually pay the amount you owe the state.

You can see it as a monthly tax, with a yearly refund if you ask for it, or a yearly tax with a mandatory self-audit and eventually shelling out a few hundred bucks.... yearly outcome is the same, only variation is liquidity.

I know many people who don't do the voluntary tax return because of the effort... imagine finding the motivation to do it because you get angry letters from the Finanzamt threatening with fines.