When shopping for a bed rack, you will see steel and aluminum being offered as material choices.
Which material is for you?
To answer that question, you have to ask yourself how, and where you will use the rack.
Advantages of steel:
Less expensive
Generally stronger
Advantages of aluminum:
Cannot rust, ever
Lighter
If you are going to use the rack near saltwater or on salty winter roads, aluminum is the obvious choice. A lighter aluminum rack may also prevent you from having to make expensive suspension upgrades. Aluminum racks are also easier to install and remove from your truck.
If you are going to be carrying heavy loads, or bomb down rough trails, a steel rack is the way to go, and it will be a little more budget friendly.