Spinach is generally considered to be better for you than lettuce, simply because it contains so many different antioxidants as well as anti-inflammatory properties which are good for things like gout and other types of arthritis. In spinach there is also a great amount of protein, vitamin A, vitamin C, and fiber along with carotenoids.
All of these things combined can make for an extremely healthy body. Studies have shown that eating spinach on a regular basis has even been able to reduce the chances of developing problems with cardiovascular disease later in life. With bioflavanoids and other important things, it is certainly a healthy vegetable which you will want to get lots of in your diet.