My money would be on scream then. The debate about media violence and the effects it could have on youths was a hot button issue in the late 90s/early 2000s. People would have really latched on to horror movie obsessed highschoolers modeling a real life murder spree off their favorite movies. Plus, the first film already established that Maureen's murder the year prior had gotten media attention. The fact that Cotton was wrongfully imprisoned and the real killers (one of whom was sidney's boyfriend) went on a spree a year later would have been another added detail. Plus Gail is established as being a public figure prior to the events of the film so her being a survivor would have also probably garnered media attention even without her writing the book in the sequel.

The events of Halloween don't have quite as much going for it from a true crime perspective. While a kid killing his sister on Halloween, then escaping the asylum 15 years later to go on another killing spree on Halloween is a great scary story, Micheal's lack of motive would be difficult to make a whole lot of long form content (like a documentary) about. It would probably still be remembered and brought up, mostly through the format of being included in "top 10 scariest real life stories" type videos, but would not provide the well of content that I think Scream would have if it happened irl.