My colleague Jennifer went to SLA and brought me back the May 2006 issue of trade magazine Streaming Media, which has a review / comparison of Captivate and Camtasia Studio. The article's not online, nor is it in any of the major aggregators. :-( Jan Ozer is the author, and it appears that he's giving a workshop on this same topic and products next month at the 22nd Conference on Distance Teaching and Learning in Madison next month.
Jan really likes the quiz options and fine control that Captivate offers, but feels that Camtasia's the better choice for everyday whip-it-out productions.
"If you're a video editor, you'll find Camtasia easy to learn and use, and Captivate unecessarilty complex. On the other hand, if you're skilled in Macromedia Director or Flash, you'll find Camtasia a blunt intrument and Captivate more intuitive and precise."
In the end, unable to see how either program bests the other across the board, Jan concludes that "it really is hard to imagine living without either one".
UPDATE: The review is now available online.