Aluminum or steel is the second decision most buyers make, after mounting style. Both work. They fail differently, and that is what should drive the choice.
Which Should You Choose?
Choose aluminum if the truck lives outside, sees road salt, or already carries weight at the rails. Choose steel if the box takes physical abuse, security is the priority, or you are buying for a fleet where cost per unit matters more than weight.
Corrosion Is the Aluminum Argument
Aluminum does not rust. It oxidises to a dull grey surface layer that then protects what is underneath. On a box that sits in weather with a gasket trapping water along the lid seam, that difference decides how the box looks in year five.
Steel boxes are powder coated, and the coating is what protects them. Where the coating is scratched through to bare metal, rust starts. That usually happens at the lid edge and around the mounting holes, which is exactly where you cannot see it.
If you run in a salt state, this is not a close call. See aluminum tool boxes.
Impact and Security Favour Steel
Steel deforms less under impact and resists prying harder. A thief with a bar gets into an aluminum box faster than a steel one of the same design. For a truck parked overnight on a street or a site, that difference is worth the weight.
Steel is also more forgiving of being loaded badly. Dropping a compressor into a steel box dents it; the same drop into aluminum can split a seam.
See steel tool boxes.
Weight Adds Up Where It Matters Least
A steel crossover can run 40 to 60 lb heavier than the aluminum equivalent. That does not sound like much until you note where it sits: high, forward and directly over the rear axle line. It is the least useful place on the truck to add mass, and it comes off your payload every day whether the box is full or not.
Finish Reads Differently on Each
On Weather Guard boxes the finish code sits in the part number: 0 is bright aluminum, 3 is brite white, 5 is gloss black, 52 is textured matte black and 6 is gunmetal gray. Bright aluminum and diamond plate are aluminum-only looks. Brite white is the fleet standard on steel because it is easy to match across a yard of trucks.
What to Do Next
If the truck lives outside, start with aluminum. If it takes abuse or sits unattended overnight, start with steel. Then pick the mounting style from the full range, or narrow straight to crossover boxes.