I went through a similar psychiatry vs psychology debate myself years ago. Chose Psychiatry. I don't regret that decision, though I also think it can vary per person.
I would suggest psychiatry if the person is not afraid of biological & physical sciences, knows ahead of time that very little of the medical school curriculum is behavioral sciences, wants to have medical doctor level knowledge of the connection between physical pathology & mental illness, wants to make more money and is prepared to deal with the reality that for better or worse the industry has become medication driven over psychotherapy driven (not saying its right, just saying the way it is).
I would suggest psychology over psychiatry if the person is not a fan of the biological/physical sciences, is not willing to go through years of intense stufy of subjects like histology & biochemistry, but instead wants to stay within the realm of behavioral science education, is willing to make less money (though its not bad in clinical psychology) and enjoys long interactions with the patient.
There are of course people who are in the middle-e.g. good at the sciences but also want to do psychotherapy. In which case you're going to have to figure this one out on your own.
The 2 fields also differ in approaches. Psychology tends to deal more in statistics & other areas of behavioral sciences such as Sensation & Perception, Behavioral Endocrinology, Abnormal Psychology, Psychobiology.
Medical School tends to deal with the more pure biological sciences. E.g. when I learned Endocrinology in Medschool it was pure endocrinology. There was hardly any psychological/behavioral correlates of the endocrinology we covered. Not surprising since it was taught for all medstudents, including those that did not want to go into Psychiatry.
Some surgery residents were really jerks who laugh at making fun of you (the med student) and the pts.
I don't think this is as prevalent in the clinical psychology culture. I'm basing this completely on my own anectdotal experience and theories. I've noticed there's a lot of narcicism & type A personalities in the medical field, especially in the more intense fields where doctors are pushed to inhumane hours of work. I think its somewhat of a defense mechanism against the difficulties & stress of the job. This is apparently is lessening & there are now movements in the field to encourage more humane approaches, but I still see it here & there.
pure acting & somewhat unrealistic but it delivers the point.
http://www.youtube.com/watch?v=LqeC3BPYTmE&feature=related
Worse case I had was an Ob-Gyn resident who screamed at the medstudents, blew up in anger several times a day, would often yell expletives (even at patients). The guy was working inhumane hours, well over the 80 hr limit, and was used to do too much scutwork. I felt a mixture of anger & sympathy for the guy.