They really have different purposes. I have both, but I use my hand mixer exclusively when I don't want to expend the energy to whip my egg whites by hand. Egg whites really benefit from being whipped by lifting the beater. This is best accomplished with a gigantic balloon whisk, but you can get almost the same results by using a hand mixer in the round bottom, unlined copper bowl. Just takes a tad of practice. It's good for whipping cream, too. Oftentimes I want a smaller amount than would whip well in the 5-quart bowl.
I also use the hand mixer when I need to carry one somewhere. Not going to cart the big KitchenAid around!