For me, it's more a matter of what can you do with each word.
With "slut" the options are pretty limited: "slutty", and that's it.
With "skank" you have the basic "skanky"; the diminutive "skankette" plus all those wonderful FReepisms like "skankster" and "skankerella".
Why choose a narrow slut when you can have a broad skank...or is that a skank broad?
Exactly my thoughts. Slut is so limiting. Skank has more possibilities.