Argentinian Neptunes
I believe the Neptunes were fitted with the APS-20, the same radar that was fitted to the Shackleton AEW Mk 2s.
As to how good the Neptunes were in comparison the the Shackletons, I reckon it was all down to the quality of the operator, as I am sure Pontious & Wenslydale will probably confirm.
During the Falklands conflict the ex-RAF Neptunes which were operated by the Argentinians were used to try to find our Task Force but I do not think they wre used in the AEW role.