I'd say the lower gpa and higher dat. SOme schools have easier versions of classes, some people lucked out with easier teachers and tests, and some give out easy As. So someone that is smarter than another person could have a lower gpa, but just had bad luck.
But with a higher dat, it shows a true standardization among all students. So all those factors that are affecting gpa aren't here.
For instance, in my O chem 2 class, the teacher used all recycled questions from old tests, so all we had to do was memorize them. We could do well even if we didn't understand a thing, so obviously kids from my class are going to do better than kids in another class whose teacher picks complicated brand new problems on tests. That's why we need the standardization of the DAT.