On the same 65.000 € gross, a worker takes home roughly 12.711 € more per year in Spain than in Netherlands. That gap reflects the tax structure alone — before rent, healthcare, or savings behaviour come into play.
Housing is the first multiplier. Spain has moderate rent pressure, while Netherlands has very high rent pressure. That keeps Spain's nominal advantage closer to a real-world advantage.
Healthcare and pensions go in the opposite direction. Spain runs a universal healthcare model — Universal public healthcare via the SNS; private cover is widespread. Netherlands uses a public model — Mandatory private health insurance (~€140/month) with regulated coverage. The country with lower take-home often shifts costs that the other country leaves to your private budget.
Net-of-everything, a relocation decision should weigh strong public welfare in Spain against comprehensive welfare state in Netherlands, plus differences in pension capture, social safety nets, and city-level cost of living.