Nothing beats a steel box for strength and 'break-in' security. But beware if you live in a northern or coastal area with road salt or salt spray. Once the paint scratches, you'll be fighting a never-ending battle against rust. That's why aluminum is so popular—it doesn't rust. An aluminum box weighs almost 50 lbs. less than a comparable steel unit. So it's easier to remove when you need the full bed. Of course, if you just want an inexpensive box to store things like jumper cables and tiedowns, you could opt for a plastic box.