I am picking 3 types of lettuce now. Prizehead is the red curly leaf growing at my community plot (center). Four Seasons is a green curly leaf growing in my home garden (right). Escarole Blonde is a plain green leaf also in my home garden (left). I also have some nice arugula I add to the lettuce bowl.
I harvest my lettuce by pulling out the entire head. I choose heads that are crowding other heads. I break off the dirty roots and leave these in the compost. I prefer to harvest entire heads because they look nice and so I can eat the tender inner leaves too. This also allows the heads that are left to grow fuller. My mom always harvests lettuce by picking to outer leaves. This works too and allows the plant to keep growing.