One mistake many people make when preparing a salad is to only use one type of lettuce. It makes much more sense to mix different varieties, because the ingredients and properties differ. Mixing several varieties such as arugula, spinach, and iceberg lettuce for example gives the body more minerals and vitamins in the same serving size. Also, in regards taste, it is a change to mix in spinach, coleslaw or even red cabbage in addition to the classic lettuce.
Metabolic Balance Blog 