Phenolic typically refers to a resin and cotton thread composite. Usually very strong and very wear/corrosion resistant. Steel is typically an alloy containing mostly iron and varying combinations of carbon, nickel, moly, chrome, vanadium, etc.... There are literally thousands of differing combinations.
A 'still is something used in Northern Michigan to make 'shine. I don't think those people will ever stop....
I would guess you are referring to brake caliper pistons. Steel is the way to go, although I have never known phenolic to cause any problem.:beer
I found it interesting when checking out Checker's line up - only the 2wd light duty truck options got metal pistons and all of the heavy duty truck options had phenolic pistons. I'm not going to worry about it, if they will last 150K, they will last forever in my trail truck.
Hater :twak :toothless You should see my grandpa's still overseas, I avoid even walking by the basement door when he runs it :shocked lol
As for the original question - with cars usually the civilian duty brakes got phenolic pistons, and the police-specs vehicles enjoy steel pistons, but there ain't that much difference between those. Generally phenolic pistons are worse for conducting heat from the pads to the caliper and consequently the brake fluid, so I'd imagine the heavy-duty trucks have them to fight fluid-boil over during extreme braking situations.