I agree with the above. Med-surg isn't the end all/be all of nursing. I started out there, but spent almost a decade working in L&D and newborn nurseries before returning to school. Now I provide primary care for inpatient psych patients.
The FNP is going to give you a lot of flexibility in your career. You can work outpatient, urgent care, specialty, and in some areas - acute care. The main emphasis of the FNP is going to be primary care - i.e. chronic disease management, minor acute illnesses, and preventive care. That's not something you're going to see a lot of on a typical med-surg unit.
The psych NP is probably going to be more financially lucrative, at least for right now. With the growing awareness of mental illness and substance abuse, I don't see a decrease in job opportunities anytime soon. You'll be confined to treating psychiatric patients, but you would still have options in terms of outpatient vs inpatient or focusing on specific mental illnesses or subpopulations - i.e. forensic, children, adolescents, etc.
Of course, if you're really ambitious, you could always get dually certified.