There is no real data suggesting one career path is better for ease of employment.
Both medical doctors and dentists are in demand. Generally speaking, either will be able to find work rather easily. Both professions offer immense job security.
Although it could be perceived that it would be easier to find work as a medical doctor, there is something to be said about the fact that dentistry is still largely isolated to the private sector.
A graduating general dentist will likely go into private practice - either as an associate or business owner. On the other hand, a newly trained physician may work for a large public/private practice or, more likely, a hospital system. Hospital systems market to large populations and can make employment opportunities known in a very public manner. Dental employment often rises out of spoken networking opportunities - family dentists, classmates, school connections, etc... It's just different.
As to your comparisons - they are fairly weak. As others have noted, comparing a general dentist to a dermatologist is a stretch, at best. However, within dentistry, a GPR trained dentist would be more competitive than one without the added training. I think this is fairly obvious, though.
With the exception of oral and maxillofacial surgeons (dental specialty), it takes MUCH longer to become a trained physician.
General Dentist - 4 years of dental school.
General Dentist + GPR Residency - 4 years dental school + 1 year of residency.
Dermatologist - 4 years of medical school + 1 year internship + 3(+) years of residency/fellowship.
Given the time of education, general dentistry remains one of the best 'bang for your buck' career paths - A general dentist with an extra year of GPR training could easily earn between 200k and 500k per year in private practice. Even if the salaries matched, the general dentist has a 3+ year head start on the dermatologist.