I went through this very same dilemma a few years back. While I'm neither an NP nor an MD (or a trainee), I can relate to you my thought process for whatever its worth - YMMV.
I started by realizing that I was interested in caring for patients on a psychosocial axis in addition to the traditional biomedical duties inherent in the role of an advanced care provider. Family/free time is also important to me, as well as my overall level of life satisfaction in the work environment (which includes time spent in training.) With these considerations, I came to the following conclusions:
1) Both MD's and NP's (or PA's) are free to incorporate all types of care into their practice. However, an autonomous MD can prioritize psychosocial care more freely than, for example, an NP who works for an MD who only cares about the "patients per hour" ratio.
2) If free time is a priority, then you can structure your practice as such. Of course, this requires choosing the right specialty, but there are more options on the MD side. Also, the same theory as Point 1 applies here - if you are at the helm, nobody can demand that you to be in the office 9-5, M-F.
3) I like working with really smart, driven people. It keeps me on my toes and, well, its a lot of fun. There is no question that it takes brains to be an NP, a PA, or an MD. However, the MD's tend to be the most driven and academically oriented of the bunch and I can identify with that. As such, if I'm going to spend years in training and a lifetime working together with my peers, it might as well be with "my kind of people." Note that its is a fallacy to think that the NP or PA path is shorter than the MD for recent graduates. This isn't true if you've been out of school or don't have the pre-req's for MD admission, but if you're already close you might as well go all the way.
I think to sum it up, you can be an MD that has a career of an NP if that's the way you want to work it. You cannot be an NP that has the career of an MD however, so if you have the chance to keep your options open I'd go for the MD. Just my $0.02 (well maybe that was $0.03 but you can keep the change).
PS. A thought on Psychiatry NP vs. MD: This is way out of my true field of knowledge, but I would imagine that the Psych NP vs. MD is similar to the Family Practice NP vs. MD. The due to lack of truly specialized training, the NP's in family practice see the more routine cases (ear ache, stuffy nose, fever of unknown origin) and refer the more complex stuff to the MD in the office (multi-system malfunction, acute or emergent cases, etc). By the same token, an NP might be utilized in the Psych world to treat the worried well, med checks, routine depression or substance abuse, whereas an MD treats the whole range of patients including those with severe psych disorders or comorbidities. I your choice also depends on what scope of practice you're interested in. Think hard, and good luck.