When I look at these questions, I ask how it will be used. IMO, the design material shouldn't matter if the rack is designed for your intended use. If you're concerned about weight and load, aluminum provides a lower weight and thus a higher capacity (assuming the same gross possible load). Welded versus not is not an issue for a well designed rack although durability is probably better for a welded rack.
A well designed rack has to take into consideration the maximum load versus the length and will define the number/size of the attachments appropriately, and the number of rails and such.
Most overlanding racks are vehicle specific and you will need to determine if your vehicle is supported by the manufacturer, or if you can adapt one vehicle's rack to work with yours.
All though I've no experience with them, Frontrunner, Gobi, and ARB have aluminum and steel racks that have overlanding in mind. These brands have multiple vendors in the US.
IMO, for a rack that will be lightly loaded (which any rack should be), aluminum is preferred but not always cost effective. An adequate steel rack that's $1000, may be $1500 in aluminum. Also, there are certain vehicles where the attachment is restricted and steel is required to mount as aluminum is too brittle for the required configuration.
