Here's an article I found that was done by Chevy High Performance comparing the results of testing the same car on both types of dynos. The article covers a lot of questions that have been raised lately on this board. Hope you enjoy the reading.
http://www.mustangdyne.com/Articles/CHP0603-article-01.htm
http://www.mustangdyne.com/Articles/CHP0603-article-01.htm