I would think it's fairly self-explanatory, the two fields seem pretty different to me (although I only have my 12 week psych rotation from MS3 as a reference point)
Derm is a field involving the skin, it tends to attract people who want a mix of medicine and procedures.
Psychiatry is a field involving the mind, I would think it tends to attract people who don't like procedures as much.
Both are lifestyle friendly although derm tends to be a more of a high volume field. In general, dermatologists make more on average but I've never met any starving psychiatrists and I'm sure there are plenty of high income psychiatrists.
There are some field overlaps with neurodermatoses and (jokingly and not so jokingly) if you see a lot of cosmetic patients.
In the end, you should rotate through both fields and the answer should come to you fairly quickly.