Tubus racks are powder coated, not painted. If you use panniers with plastic attachment points it'll likely take a long time before they wear through the powder coat. If you want to protect the rack at all costs, you can apply some frame protection stickers or other protective tapes just the way you'd protect your driveside chainstay on your bike frame.
I believe Tubus is considered the gold standard because their weight to carrying capacity is better than the equivalent aluminum rack. The Old Man Mountain Classic Rack (aluminum) weighs 1134g and can carry up to 70lbs. The Tubus Cargo Evo (steel) weighs 780g and can carry up to 88lbs. The Blackburn EX-1 (aluminum) weighs 535g and can carry up to 45lbs.
Despite being susceptible to rust, steel racks often last longer because they have a much longer fatigue life than aluminum racks. If your rack does crack on tour, both metals can be welded together, but the aluminum weld will be much much weaker unless the rack is heat treated afterwards.
Either material is fine for making racks but I'd personally stick with steel for piece of mind.