In the theatrical cut, Michael breaks away from the cult and attacks it inside Smith's Grove, Tommy and Kara carry the children out of the building, and Dr. Loomis stays behind with his fate left open. The film leaves Michael's story unresolved, and it sets up the later sequel Halloween H20.
What does the cult actually explain? It turns Michael from a force with no stated reason into a man controlled by Dr. Terence Wynn and bound to the curse of Thorn. That is a large retcon. The original Halloween 1978 offers a boy who kills his sister and a man who escapes the sanitarium fifteen years later, and the sequels through Halloween 5 leaned on family ties and Jamie's mental link to him. Here the family story is replaced by a ritual one. In my opinion, that swap is the film's biggest gamble, and it is also why the sanitarium finale, with Michael turning on his handlers, reads as the point where he is no longer anyone's instrument.
How does the Producer's Cut differ? It is a workprint with about 45 minutes of alternate footage. Jamie survives the opening, and Loomis takes on the cult's curse. The theatrical version came out of a test screening in New York City, after which the ending was rewritten, reshot in Los Angeles in summer 1995 and the film heavily re-edited. The two cuts are close to two arguments about what the curse means, one where it drifts off with Michael and one where it lands on a named character.
Which version is better? It could be argued either way. Perhaps the Producer's Cut gives Loomis and the cult a clearer purpose, because the curse passes to someone the audience has followed since Halloween II 1981, where he hunted Michael through Haddonfield. The theatrical cut runs 88 minutes and keeps its attention on the chase to get the children out. If you want the faster film, start there. If you want the version in which Loomis takes on the curse, the workprint is the one to track down.
Donald Pleasence died on February 2, 1995, and the reshoots took place that summer, so the open ending for Loomis in the theatrical cut reads, perhaps unintentionally, as a farewell to the actor as well as the character. It was his final film.
Where does the curse go afterward? Nowhere. Halloween H20 ignores films 4 to 6, so Thorn, Steven and the Strode household of this film do not exist in that timeline, and Michael returns as a brother hunting a sister who faked her death. The cult storyline is a dead end by design, which is a useful thing to know before you place this film in a marathon.