I always thought of it as one will probably **** and the other might or might not ****.
YMMV.
Hmmm ...
synonyms never mean exactly the same thing
A slut almost certainly will ... her appearance may or may not be disheveled.
A slattern is, by definition, disheveled in appearance ... she probably will, but it’s less certain.
Check out the word history for both terms at “dictionary dot com”.