This type of pizza could do well with or without sauce. In fact, a lot of spinach & feta pizzas I've seen use fresh tomatoes instead of a sauce. A standard tomato sauce with seasonings might in fact obscure the delicate flavor of the spinach. You could also try a non-tomato sauce such as pesto or ricotta. I once made a white pizza with sauteed spinach, ricotta, and feta that was fantastic. If you go with a tomato sauce, I would use an uncooked sauce made from pureed tomatoes and go easy on the sauce seasonings.