The autopilot comes out on top here because it doesn’t have an instinctive aggressive response
Extract from the B737 FCOM Supplementary Procedures Adverse Weather:
Severe Turbulence.
Yaw Damper....On.
Autothrottle… Disengage.
AUTOPILOT....CWS.
The problem with hand flying in severe turbulence is the risk of pilot overcontrolling. Pilot induced oscillation is a common characteristic and this can be critical at high altitude. Automation dependency can make things worse where reversion to hand flying by pilots lacking the necessary competency often leads to pilot induced oscillation. Some pilots are worse than others.
Large and rapid movement of ailerons not only causes unwanted spoiler operation but throw in unnecessary movement of the elevator and rudder in response to large scale excursions caused by gusts, and the stage is set for pilot induced unusual attitudes. Hence the Boeing 737 advice to use autopilot CWS where the pilot is still in control but the controls are far less sensitive and more resistant to inappropriate manual inputs.
So it all boils down to individual pilot manual handling skill; especially in severe turbulence encounters in IMC. Instrument flying skill is paramount and some pilots don't have it. . Nowadays, where the accent is more on full use of automation than basic handling skill, it would be wise to accept the autopilot can usually do a better and safer job than a rusty pilot wrestling with the controls.