the two ICs are for different purpose, as stated in the datasheet the MAX483 is slew rate limited, that is if you don’t need fast communication will not emitt much electronic noise (through the wires) as it has its spectra limited like the FFT diagram shows. MAX485 on the other hand will handle faster bitrate, but this is only possible with using faster edge transitions which may be emitted to the surrounding circuits. It is the designer’s duty to select which one suits the application more.