There was a time when these terms were readily understood. Their scope was nicely defined. Essentially both were the terms for a person well versed in the world of Sherlock Holmes. This world was generally understood to compromise the canon of stories,tv adaptations. Film outings,plays,scholarly writings and,to a certain extent, pastiche efforts.
the more serious members of these ranks were also members of related societies and, perhaps, contributed scholarly articles to society journals.
The principal difference between the two terms was that Holmesian was decidedly British (and formal)and sherlockian was used across the pond.
since the BBC’s Sherlock had taken hold, the term Sherlockian has been appropriated in a way that Holmesian had not. Social media is full of people who describe themselves as Sherlockian. However, a significant number (not all) of these will have done no more than watched screen adaptations. In the uk at least, Sherlockian had effectively become a term for a fan of the BBC’s series.
The two terms which were once separated by geography rather than scope, are now effectively separated in the uk at least by scope rather than geography. 🤔