In terms of comprehensiveness alone, the MCAT destroys the DAT. I can't speak from experience, but the DAT is more straightforward in terms of testing style. From what I'm told, if you know the material, you'll score well. Also, their prep books are no where near as lengthy as what most MCAT prep books cover. With the MCAT, because of the emphasis on reasoning and logic, knowing your stuff well is only a fraction of what's needed to score well - part of the reason why people with high gpa's bomb the MCAT because they're oblivious to this.