as someone who was in a similar situation a year ago (EM vs. anesthesia) i can give my biased perspective (as i chose and just matched into anesthesia)
1. my personal feeling is most ER docs in their 50's in 60's just seem "tired". I've spent about 12 weeks of clinical time in both ER and Gas, and have meet several anesthetists in their 60's and even early 70's, but have seen few ER docs in their 60's (maybe one?)
2. I've meet many people within anesthesia who were either deciding between the two as a medical student and choose anesthesia ultimitely, or switched from ER to Anesthesia during residency, or retrained in Anesthesia after working in ER fore a few years.
I think i might know only 1 medical student who choose ER when deciding between the two, and maybe 1 ER doc who considered anesthesia as a medical student but decided on ER eventually because he thought anesthesia would be too boring for him.
I don't know ANYONE who has transitioned from anesthesia residency or practice to ER (although if you look hard enough i'm sure you could find someone). I think this says alot about the lifestyle and long term tolerability of the two.
3. I chose Gas because i feel there are alot more procedures, less social issues to deal with than ER, generally higher acuity overall (i don't like the GP side of ER- prescription refills, cough's and colds etc.), really into phys and pharm, and can see myself doing it when i'm 50 or 60 (not so sure i would be wanting to do ER in 50's and 60's), and hours seem better for when i have a family in the future.
I used to think the hours in ER were pretty sweet, but i think once i have a family in the future, i won't be so cool being away from home in the evenings and overnight.