It's not much of a distinction mathematically, but it's an important distinction in emphasis. Saying something is growing exponentially does imply that it will just keep on growing, at least for a long while. Saying growth is sigmoidal emphasizes the eventual slowdown.

Talking about "exponential growth" often leads people into unrealistic hyperbole. Of course, by "people" I mean marketers, investment advisors, and headline writers at Business Week...not us rational computer scientists!