Both Spinach and Lettuce are two entirely different vegetables and Spinach and Lettuce are not even in the same family of vegetables.
Spinach also contains more Potassium, protein, calcium, iron, niacin and vitamins A, C, B, C and B-12 than Lettuce does which makes Spinach more nutritious than Lettuce.
But both Lettuce and Spinach are healthy vegetables to eat and are nutritious.