Should we spend an extra day in San Sebastián or Bilbao?
Bilbao
It depends on where you´re coming from and your interests. Very different cities but also very similar in many aspects. Donostia-San Sebastián is more like a postcard city. Not a beach resort of any kind, it´s a working city with three beaches and a great gastronomy. It can be easily seen in one day, very walkable. Bilbao offers the Guggenheim museum, an architectural jewel, and a beautiful old quarter, also with great gastronomy (that´s the standard for all the Basque Country, IMO). Bilbao has probably more things to see and do, Donostia-San Sebastián is more a city for leisure and relax. Both are located in a very rainy area (although this year it´s been quite dry), weather is very unpredictable.