My answer: From my research I think it’s mostly preference. Vidalia or Walla Walla Sweet onions are obviously sweet onions and I don’t care for them in cooking something like soup, stew or Rouladen (they’re simply too mild), yet they have a wonderful crunch and flavor in salads or on burgers. In some instances purple onion will change the color of delicate sauces, which might prove to be undesirable. Something I’ve observed in recent years, after living in Washington, New Mexico and Texas, is that onions can be stronger and/or milder depending on the region. I’m not sure if this is due to storage procedures, soil differences, shelf life or local temperatures (or a combination) that make the difference; at any rate, onions are not created equal. Here’s an article on nutritional value of purple and yellow onions and Onions, Layers of Flavor is an article with almost everything one might want to know about onions. Thank you, Mae, for your question and your interest in Ask Cynthia a Question.