There was a time when these terms were readily understood. Their scope was nicely defined. Essentially both were terms for a person well versed in the world of Sherlock Holmes. This world was generally understood to comprise the canon of stories, television adaptations, movie outings, plays, scholarly writings and, to a certain extent, pastiche efforts.
|A learned journal|
The principal difference between the two terms was that Holmesian was decidedly British (and perhaps formal) and Sherlockian was used across the pond.
In the latter stages of the 20th century the term Sherlockian began to be used more and more by British devotees to the extent where the two became interchangeable. It was, however, generally the case that the users of the American designation tended to be younger than the adherents of the British one.
Since the BBC's Sherlock has taken hold, the term Sherlockian has been appropriated in a way that Holmesian has not. Social media is full of people who describe themselves as Sherlockian. However, a significant number (not all) of these will have done little more than watched screen adaptations. In the UK at least, Sherlockian has effectively become a term for a fan of the BBC's series (and possibly the recent RDJ films) and Holmesian remains largely the term for the more scholarly devotee.
Given that usage is the ultimate arbiter, it seems that these 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.