Overall both are imp. But lets say you have a low undergrad GPA with everything else that looks good on your app. A strong DAT will help you compensate for the low GPA. If you have a very high GPA )3.8-9) and avg DAT of about 18-19 works the same way.
But rmr take into consideration: ECs, LORs, Major, Community service, leadership, etc.
Each person's case is always diff. thats why we have so many sdn threads of ppl asking if they have a chance at certain schools and what they should do.