AP, the standard autopilot on the C402C was a Cessna 400 series which, for their age, work surprisingly well. That was what was fitted on the C402C that I flew aeromedical work with and all modes (heading, navigation & altitude hold) worked well. On the three C404 that I am currently flying two are fitted with the 400 series autopilot and one with a Bendix-King. One of the 400 series works ok in heading mode, doesn't work in navigation and altitude hold is only so-so. The other 400 series and Bendix-King are excellent.
I have also flown a Baron and a Vulcanair equipped with S-Tec 55 autopilots. The Baron was excellent, the Vulcanair was appalling when it came to altitude hold but I suspect that the problem was caused by the interface between the servo and the aircraft.
A summary, both are good but the S-Tec is probably going to be better for a longer period. The 400 series autopilots are getting long-in-the-tooth and will require regular servicing.