I initially opted for post categories, because that made most sense for the jekyll-archive plugin. That plugin is not supported on GitHub pages, so I implemented categories as a filter for posts (see e.g. http://peterdesmet.com/petridish/blog/?category=Shared+category).
The order does not matter (in categories, those result in differently stacked subdirectories when using default permalinks)
It does not conflict with how posts are organized in directories (e.g. horror/_posts/ would automatically add the horror category, but not a horror tag).
So even though this is a breaking change, I'd prefer to use the one that conceptually makes the most sense. It also frees up categories to be used differently.
I initially opted for post categories, because that made most sense for the jekyll-archive plugin. That plugin is not supported on GitHub pages, so I implemented categories as a filter for posts (see e.g. http://peterdesmet.com/petridish/blog/?category=Shared+category).
However, that is conceptually more a tag than a category in my opinion:
horror/_posts/would automatically add thehorrorcategory, but not ahorrortag).So even though this is a breaking change, I'd prefer to use the one that conceptually makes the most sense. It also frees up categories to be used differently.