I just switched from ob to psych and think that I will go into reproductive psychiatry. It's not so strange a dilemma. I agree with the above poster that ob/gyns are not very comfortable prescribing much more than an SSRI. Any other class or any polypharmacy and they try to send the patient to a psychiatrist. Unfortunately, most psychiatrists don't like to prescribe to pregnant patients. So you end up with a group of people with very poor access to health care at the time when they need it the most. If you want to help pregnant patients with their psychiatric needs, go into psych.
However, if you want to put your hands on your patients, you should go into ob, because you basically don't do that at all in psych. Look into what it is you like about each field and see if you'd like to be doing that for your career. If you hate gyn surgery (which I contend no student knows, you really have to perform surgery to rule it out), you can still do ob/gyn, just go into mfm (and take care of mentally ill pregnant women--there are mfms who do this), but you will still have to do c-sections and sometimes c-hysts and take care of non-psychiatric high risk pregnancies as well. As someone stated above, your focus will be different coming from those very different backgrounds. What you can offer the patient and how your relationship will be defined will also be very different. It really depends on how you want to relate to your patients.
Taking lifestyle into account is also a good idea. 🙂 Good luck.