We use synthetic material for our press beds (Polymetaal presses, Netherlands) www.polymetaal.nl
It is cheaper, safer, lighter, does not rust, does not warp, easy to clean, and the surface is white.
However, if you prefer steel, you should use hot rolled steel, that has been surfaced (milled and ground). Cold rolled steel looks nice, but because of the cold rolled proccess, the steel has huge tensions inside, that come out slowly afterwards, resulting in warping of the press bed. (wether you work on it or not)