I've taken both exams and have to say that neither can be called 'harder' than the other, it just depends on your strengths/weaknesses.
Sciences: w/regards to orgo, g.chem, and bio, both exams require the same level of mastery of the material; there are some differences, no doubt (e.g., in bio the DAT has more emphasis on taxonomy and MCAT on physiology), but the the 'level' of difficulty is the same; However, a particular question requiring mastery of the same material can be harder on the MCAT b/c it requires a more 'conceptual' understanding, an ability to integrate the cold facts with other bodies of knowledge.
Yes, the MCAT has physics but then, the DAT has math and the PAT. The latter, especially, could be much harder for some than a regular subject such as physics. It's a very visually demanding subject where you can't rely on memorizing formuals and crunching numbers.
The reading/verbal sections are pretty similar. The DAT stresses more how much you can recall specific details from the passages while the MCAT is more concerned with themes. The DAT passages are also mainly medical in nature, while the MCAT ones can just as easily be from the humanities. The MCAT has writing to boot, but then your score on that section is more of a side note for schools than anything else.
If you can get a science avg. above 21 on the DAT that should easily translate into a 10 and above for the MCAT (and vice versa)...I've known a few people who've taken both and this scale has always been true (assuming equal study effort has been put into both).
Good luck whatever road you take!