What are the differences?
Don't know too much about medical school, though I do know that several dental schools have their students take various classes with their medical students during the first 2 years (the didactic years). Dental school requires much more lab time/work as far as I know, and much of the clinical portion of the curriculum is spent at the school, whereas much of the clinical portion for med students is spent at hospitals.
Which is tougher, which keeps you up at night the most?
Impossible to say, few people have gone through both medical and dental school, though there are a few oral surgery residents here that attend medical school now, they could probably give some insight. From what I've heard, both require late nights studying, but it highly depends on what kind of student you are and how badly you want to specialize (ie how much effort you put into studying).
How are the class schedules different?
Class schedules are 5 days a week, how they specifically differ between medical and dental school would be based on specific schools. Like I said before, at a school where med and dent students take classes together, the curricula might be similar, but comparing two different dental schools, or a dental school at one school against a medical school at another school, they could be extremely different. I recommend looking at the curricula for both at a specific school, say Columbia or Harvard (I only use these as examples because I know they have several combined classes for their dental/medical students).
Is it harder to specialize into Dermatology or Ortho?
Both are difficult to specialize into, you'd need to look up statistics on applied/accepted for both. Honestly though, they're so vastly different careers, why even compare the two? Are you actually interested in doing both? My point is it shouldn't matter which is more difficult to get into, it should matter which you're actually interested in or more interested in for what you'll be doing.
If I was looking for more of a better lifestyle; less hours, no calls, relaxed environment(non hospital), would Dental make much more sense?
Yes.