A bit simplistic, but in a nutshelll....
They are used for different things. A natural PK is used for data
integrity purposes, and if it is a single attr, it can also be a
performance enhancing index.
An artificial key is used to speed up queries, by allowing joins on a
single indexed attribute.
So use your composite, data driven, unique index as an integrity rule to
prevent duplicates, and use an artificial PK to enhance performance on
queries & joins. When you need to enhance performance is largely up to
you, unless you require your design to always maximise efficiency.
HTH,
Brent Wood
