This is obviously a biased view, but many of my firends/classmates did go into Anesthesia so I know some of their reasoning.
Anesthesia:
Pros: Lifestyle, hours, pay, and benign residency.
Cons: Working with surgeons, little patient interaction, and lack of respect from surgeons and sometimes nurses -- this does not occur frequently, but when I did my GenSurg, Ortho, Neurosurg rotations, I did get that feeling.
PM&R:
Pros: Lifestyle, hours, pay, and benign residency. I liked working as a team with ST, OT, PT, etc. PTs and OTs are pretty cool and fun to hang out with too.
Cons: Most medical students or Pre-meds know little about the field since there are very few PM&R residencies and even fewer good residencies. I think the main con is that their is a lack of understanding about what a Physiatrist does.
Why I chose PM&R?
I didn't really like to work with surgeons. I am interested in Interventional Pain Managment, but that's not all I want to do. I wanted to do a little bit of Sports, IDETS, SNRBs, and EKGs. My wife is an OT so I knew much more about PM&R than most medical students otherwise I might not have known about the field and would've chosen something else. I also plan on opening my own clinic and the only way I could do that with Anesthesia would be to open a Pain Clinic and that would get boring and frustrating after awhile. Also with an aging population and very few PM&R residencies, the outlook looks good for a physiatry -- especially for someone who comes from a good program.