I hear it the other way, Brass are soft and won't last as long as Steel, the plugs won't rust be will get weak and leak before a Steel plug will-so long as theres no rust.
Brass is for racing motors which get broke down each season--the Brass plugs come out easier. Steel is for street motors that might go 10-15 years without getting pulled. Brass will work in a street motor as long as you don't drive it for 100,000+ miles--like many are.
Factory used Steel, some are 35 years old or more, have 200,000 miles and are not leaking. You need to change the anti-freeze no matter what type of plugs you use. Paint on the outside of the motor stops the Steel from rusting.
