The Basque Country and Navarra do not apply the state scale plus a regional one: they have a complete income tax of their own, under their foral system. In the Basque Country, each province (Álava, Bizkaia and Gipuzkoa) has its own law.
The main differences
A single scale, starting at 23% in the Basque Country and 13% in Navarra.
Tax credits instead of personal allowances: a general credit per taxpayer plus others for children, dependants or age.
Their own work-income relief: the Basque Country has a work-income bonus; Navarra uses a tax credit.
For autónomos, their own percentages for hard-to-justify expenses.
It depends on your salary and family situation. On low and middle salaries, the foral credits usually leave more net pay; on high salaries, the foral scales can be steeper. Each territory's page shows its full scale.
Social Security, on the other hand, is the same across Spain. Work out your foral case in the calculator.