Both Arugula and Spinach are great for your health but Spinach contains more health benefits than Arugula does.
Spinach contains more calories, Protein and Vitamins.
Spinach also contains more of the vitamins K, C and A as well as Folate, Potassium, Manganese, Magnesium and Iron which helps to boost your health.
I personally like to eat both Arugula and Spinach together on Salads so by doing so I get both the health benefits of Arugula and Spinach and for me both makes a great salad but different peoples tastes vary.
But in short simple terms Spinach wins in the health benefits side with Arugula being second of being healthy.