This choice depends on the primary function:
42CrMo (1.7225): Selected when the component requires a combination of high core strength/toughness and good surface wear resistance (often achieved by surface hardening). It's a structural steel. Used for gears, shafts, spindles-components transmitting torque and bearing bending loads.
100Cr6 (1.3505, SAE 52100): A high-carbon, high-chromium bearing steel. Its primary purpose is to provide maximum contact fatigue resistance and wear resistance in rolling element bearings. It is through-hardened to very high hardness (~60-64 HRC) but is relatively brittle. Its core toughness is much lower than 42CrMo.
Selection Rule: Use 42CrMo (+QT, often surface hardened) for loaded, rotating components. Use 100Cr6 for bearings, rollers, and balls where rolling contact fatigue is the dominant failure mode.