I'm a mechanical engineer and just about everything structural that I design gets made out of mild steel by default. If weight is a big concern, I consider aluminum. If strength is a big concern, I consider a higher grade steel. If the coating getting worn off is a big concern, I consider stainless. But if none of those are an issue, mild steel is the way to go. Cheap, easily weldable, easily machinable, and pretty damn strong.
By the way, stainless doesn't mean rust-proof, just rust-resistant. Leave an uncoated piece of stainless out in the rain for a couple of days and it will usually develop rust spots.