SarOwl - the Sea King 3A has a duplex FCS since each VG feeds a lane in the ASE unlike the Mk3s and 5s and it has comparator circuits to monitor each lane. It has the SN500 FPC which should be familiar to S61 drivers as I believe they have the SN550 fitted for SAR work.
The 3A also benefits from the RNAV rather than the CDNU which brings better connectivity so it can drive the aircraft around search patterns and nav routes just like a real autopilot. Additionally the RadOp can send marked targets forward to the RNAV from the radar.
I think the 139 is just duplex but benefits from an inertial/GPS nav system that doesn't rely on doppler for the AHT functions. I didn't particularly like the TD on the 139 - it was quicker but steeper than a Sea King which felt uncomfortable in light winds at night. The Sea King profile with a gradual reduction in speed which reaches set height before zero groundspeed (or the 10 kt trickle in a 3A) is more comfortable in IMC.