This chart compares the nutrient values of Kale versus those of Spinach. They are both great foods and I would encourage everyone to include them both in their diets!
Spinach is slightly higher in everything except Vitamin C and Kilojoules, but Kale seems to get all the glory at the moment. They are both great choices, just like all the other vegetables and fruits.
A super diet is far more important than individual foods and a super diet should include a variety of fresh produce every day.