Sweet onions have less sulfur in them and that makes them less pungent tasting and smelling. Since they lack the spicey oomph regular onions posses, it's best not to substitute them in recipes. 

Cooking them tends to take away from their beautifully complex and more mild flavors. Try them fresh in a fennel, grapefruit salad. OR try in an avocado pasta salad, yum!