From what I have seen,
Surgery residents work their asses off, but are happy, because all they want to do is cut.
Anesth residents are happy 'cause they don't work as hard, get to do medicine stuff, get to do procedures, don't have to round (except in specific and infrequent rotations like ICU), have very rewarding pt interactions because they help a pt through a difficult day, and it is very common to have the postcall day off. And, most anesth staff docs are nicer to them than surgical staff docs (just compare M&M conference atmosphere). Also, when an IM decides on a med therapy, gives the pt a pill, they have to wait a couple of weeks to see if it was a good idea (ie HTN meds, or other such). In anesthesia, we see our drugs work in 45 seconds. There is much more immediate gratification.
hours.
IM residents work as hard as surgical residents, but are an unhappy lot from being dumped on by every other specialty but anesth. How many times have you seen Ortho service have a pt with a fracture, pt has HTN or a raised Creatinine, and they admit the pt to Medicine so all they have to do is deal with the fracture. I'm just saying...
Again, don't take this for anything more than my opinion and observations through the years. There is a reason we very often see people change from IM to anesth (or after completing IM, they do a second residency in anesth because they are sick of the IM life). I have rarely, if ever, seen an anesth resident change to IM, unless forced to because they were not doing well in anesthesia residency. I admit I have seen that twice.