In general, the default choice should be an alias. An alias should be used when it's likely that anyone will still use the old tag. This depends mainly on how popular or well-known the tag is, not strictly on its size. Size is a factor, but it's not the only factor. An artist can have a large following even if they have a small number of posts.
The main reasons not to use an alias are:
- The tag is ambiguous and an alias would cause confusion.
- The tag could possibly be used by a different artist or character in the future.
- An alias would get very little use.
- An alias would create unnecessary clutter in autocomplete or in the wiki.
Sometimes people think that aliases should be avoided because too many aliases could strain the website. This isn't really true. Aliases are basically free. It's more about not accumulating a bunch of aliases that nobody uses.
When it comes to manual moves, please don't move well-known tags without discussion. This especially applies to artist tags. I sometimes see people moving well-known artists with hundreds of posts and it drives me nuts. We have literally thousands of broken saved searches now because of people moving artists manually. In many cases these moves aren't even necessary, they're just fiddling around with artist qualifiers. I have a huge backlog of saved searches I have to fix now because of this.