We'll have to clarify which PM is private message, which is preventive, and which is palliative
But, I'll go with preventive for now unless otherwise specified... Your post reminds me that my post from 2008 is not really correct anymore:
Prev med residencies are not quite the same. As mentioned, PGY1 year is clinical (and would be fulfilled by the poster's PMR residency). Year 2 is academic (getting your MPH) and year 3 is basically a practicum year. And, many in prev or occ med are dual boarded (in IM, peds, family, PMR, etc).
This was correct when I was a medical student applying to IM residencies. Several years ago (I can't remember when, but probably 2010 or so, based on my timeline), ACGME changed the "rules" for PM residency to make our training more consistent with other clinical specialties. Specifically, most places now require that you have some of your "clinical" training consistently through the 2 years of training (still PGY2 & PGY3). So, the old idea of just doing your MPH in one year then one year of practica / rotations is no longer accurate. Most programs I know of switched to doing both your coursework and your rotations at the same time. But, as our rotations are a mixture of public health and clinical (and tend to be longer commitment than 2-4 weeks like IM rotations). The main reason I wanted to point that out here is that, under the "old" way, one way to streamline your length of training was to have your MPH before hand. If that was the case, then PM training only required that 1 year of rotations. But, now... I'm pretty sure... you won't get out in less than 2 years. There may be nuances I don't know, though (for example, I know that Mayo's training program is technically a "fellowship", but I don't know if that affects your standing for being boarded - I presume not, but we could check). Also, there are some combined programs, but very few. I *don't* know about the ease or availability of transferring in to a combined program. But, the Griffin program may be one to look at (and I can point you to the right person at ACPM who keeps track).
My long-term mentor was an IM/PM doc, and his advice at the time for my career goals was invariably that I should do IM first (if I wanted to be a clinician). He also voiced concerns about the flexibility and status of PM as a field. As you can tell - we don't have a forum on here. I later learned through ACPM that many people never even know that PM is a career option because their dean's offices don't know about it. We don't participate in the match (for the most part), and you have to at least one year of clinical training before you apply, so relatively few graduating medicine students even have that on their radar. And, PM residencies are not funded by the same mechanism as everyone else (CMS), so it can be a real challenge. But, we are a boarded specialty and have been for decades. Occ Med & Aerospace are our "sister specialties", and PM docs can sometimes go the OM route (and get boarded as such). There are many opportunities for dual-boarded folks. And, there are definitely great career opportunities being only PM-boarded. But, not having the "primary care" specialty under your belt simply does limit you.
My anecdotal take on job searching now... There are 10 X as many OM job posting as strictly PM positions. And, probably 10 X as many IM positions as OM. And, like I said, I don't think most IM/FP fellowships will take you just being PM trained. We have recognized fellowships, of course (toxicology and informatics being the most obvious). And, you can certainly have a clinical career being PM-boarded only. But, there are also a number of "less" clinical options (I'm not going to say "non-clinical" but less clinical than I was hoping for), so I don't want to paint the job market as unfavorable.
But, being dual-boarded really makes you a rockstar when it comes to options. And, like you said, having two years of intensive, essential "on-the-job" training in PM gives you an excellent skill set for epidemiology, health systems / administration, community / public health, and clinical prevention, which I think is worth it. And, lastly - PM & OM docs tend to have some of the highest reported career satisfaction, for what it's worth