WEBF
Both.
The Mk5 upgraded the SK Mk2/Wessex active system with a deep water mod, meaning extensive winch and pithead gear mods, in addition to the new Sea Searcher radar and TANS. But it wasn't all integrated, which was the main thrust of the Mk5-6 upgrade, along with secure comms.
The existence of the short-lived Mk5 is slightly misleading. The two conversions were actually pre-planned phases of the same programme, with Mk5 and Mk6 monies released for development at the same time (early 80s). The new AEW Mk2 for example was the first to get the Mk6's secure comms, albeit in a poor Service Mod. 'Mk5' was a configuration control nicety, as that build standard became the baseline for the Sea King drawing set, not the Mk1. (So, the AEW Mk7 was in that sense a mod to the Mk5).
Even Mk6 was meant to be short-lived, as Merlin ISD was meant to be 1989, and indeed most of the avionics were ready by then. With delays to the airframe, the Mk6 sonics Active Hardware and software was upgraded every 18 months or so - as I said, essentially to keep the Merlin kit up to date. Issue 5 was 1994, and the last was Issue 6, which was software only. But one thing Mk6 didn't get was its intended expanding array sonar. It was all trialled in about 1985, but it was decided to wait for Merlin. I lost track of it after that.
It was quite difficult keeping track of what was fitted to what, and as Xmit said a cab could externally look like a Mk5, but be a 2.5 inside. The ructions caused by having to shut down development and launch production up to a year early in 1982 was never really resolved in the life of the ASW SK. Much of the pre-requisite and risk reduction work for the AEW Mk7, in 94-98, was stablising this build standard, but only for AEW. (The equivalent work on Nimrod wasn't done, which is ultimately why it had to be scrapped. Another story).