Well, I was definitely torn between the two. Psych was always my first love but I am an exercise phys and rehabilitation guy by avocation and necessity. I ultimately stayed with psych, although I'd be lying if I said I never wondered if I made the right choice (granted, I'm only 3 weeks into residency lol).
Both of you covered why they really are so similar. They are the only two fields in medicine in which we seek to help the patient heal themselves. Sure, every specialty gives lip service to lifestyle change and prevention, but these are the only two fields that roll up their sleeves and get their hands dirty with the actual act of helping the patient put it into practice.
I still intend on making a huge focus of my practice and research working with people with MSK/neuro issues. And ultimately I felt like psych was a better vehicle for that.
I've been a MSK/neuro patient myself for almost 12 years (parsonage-turner syndrome with involvement of superficial back muscles...leading to becoming a walking textbook of spinal pathology and si joint, wrist, elbow, and shoulder issues). I don't look it. Or act like it. And although call nights are so bad I'm swaying on my feet from pain and myelopathic leg weakness half the night, I have no clue what taking an opiate feels like.
I've seen my share of back pain and neuro patients. And while some do great despite their physical pathology (I'm a chump compared to say...Tim Champion, Master Hinkle, or Nick Scott), most don't. Some of this of course comes down to basic genetic and constitutional resilience, pain tolerance/sensitivity, and muscle genetics. But that still leaves a huge gulf between your 'average' chronic MSK/neuro patient, and the exceptional.
Rightly or wrongly, my personal experience, the testimonies of the people who put me to shame, and hearing the stories of others with similar debilities who've allowed their physical injuries dictate their lives, have led me to believe--rightly or wrongly--that much of the difference is psychological.
Which is ultimately why I ended up choosing this path. I felt like I could do more with these patients by helping them get their minds right, so to speak. Mindfulness and breathing-relaxation techniques to decrease pain, to break the cycle of pain-spasm-pain-spams that can leave your whole back locked up as a single unit, to keep them from becoming dependent on opiates and muscle relaxants. Cognitive approaches to help them recognize pain as a signal of underlying dysfunction. To help them see the dysfunction as the real enemy, not the pain. And to help them recognize that not all pain is bad.
That said, there is a lot of work and a lot of research on the physical side of things that needs to be done. And we need inspired and open-minded people in PM&R now more than ever. We are a far, far cry from having optimized approaches to rehabilitation for a lot of MSK/neuro issues. I hit the limits of what conventional pm&r could provide as far as rehabilitation a long, long time ago. I got where I am by a combination of stubborn bloody-mindedness (which both hurt and helped), and having to resort to going back to first principles and designing my own rehabilitation/PT protocols from first principles, reading the kinesiology and exercise phys textbooks on my own (and no small amount of help along the way from Cressey, Boyle, and Robertson...none of whom are physicians).
Sorry for the ramble. And I'm sure that sounded extremely egotistical, but *shrug* hopefully you get something out of it.