It all depends on the lettuce. I grow a mix of leaf lettuces. I believe fresh (reduceses saminella bacteria) organic (I can control whats in the lettuce) and type are good reasons to grow. Lettuce is easy to grow in cool weather. I can not grow it with the heat here now (92% today)

but will be sowing more seed soon to grow true the winter. They like oak leaf lttuce, ruby red lettuce, blackseeded simpson, endive, arugala, and muslic mixes. I make my own compost for fertilizing organically. Also some spinach and any kind of greens like collards, kale, and of course they love carrot top greens.
Much of the lettuce bought from stores are grown hydroponically (in water with chemical fertilizers) and just aren't that nutrious. If you must buy from the store it is best to buy organic and stick to the kales, collards, broccoli...all are high in calcium, magnesium, and other nutrients.