I took both DAT and MCAT. I took DAT first, MCAT a few months later. I can go through it by MCAT Section
Chem/Phys
I think the section where the DAT was most helpful was chemistry. DAT covers general and organic chemistry pretty in depth. In fact, I'd argue it goes more in depth than the MCAT on some areas, like orgo reaction mechanisms. I don't think I had to do any additional orgo review, and only marginal gen chem review when it came time for MCAT content review.
DAT doesn't cover physics at all, so there was no overlap there. I'm not a strong physics student, so relearning all the formulas was tiresome and took a significant amount of time.
Bio/Biochem
DAT is physiology heavy, so it prepared me well for MCAT physiological systems questions. That said, there's divergence in terms of concepts covered between the exams. For example, DAT had a stronger focus on ecology and evolutionary biology (I think I even remember photosynthesis questions on some practice exams.) MCAT doesn't have that. MCAT does, however, expect concepts from microbiology and immunology that aren't present on DAT.
Moreover, DAT has very little biochemistry on it. While I was familiar with pathways for DAT, MCAT requires a much higher level of comprehension.
CARS
DAT Reading Comprehension preparation should help you with MCAT CARS. I think DAT Reading was easier, because generally DAT questions were quite literal and would align chronologically with the passage. CARS doesn't do that.
Psych/Soc
DAT does not have this section, so your DAT prep will not be helpful here. That said, there are many online resources for self-studying this material.
Overall Assessment
MCAT is harder in terms of breadth and depth of information. More than that, though, what I found most challenging about MCAT was just the format of the exam. DAT is made up of entirely discrete questions. The questions don't actually require that much thought, as as long as you've memorized the correct information. Conversely, the majority of MCAT questions are passage based, so you need to not only know the information, but be able to recognize and apply it. Plus the sheer amount of reading on the MCAT makes it much more tiring than DAT.