I think the choice should be determined by the type of end users of your system. If it will be used by people who know the slang, go for slug, it is used in many CMSes.
If the software is designed for someone whose mission is to write text, and who are not familiar with tech-speak - then "slug" is a poor choice, because it rings no bells about the meaning of the feature. One will only figure it out by reading the manual, by asking someone else, or (most likely) by trial and error.
So my point is that you should go for a name that expresses the meaning of the feature, ex:
- friendly URL
- memorizable URL
- permanent link (something that is there forever, so it better be good)
I would also consider changing "URL" to "link" or "address", because "URL" could mean nothing to a newbie.
Regardless of your choice, you should:
- fill it with a reasonably good default value (generated from the entered title)
- show a preview
This enables people to understand what this field is for, without submitting the article and then observing what happened.