I started med school thinking I would do emergency medicine. Currently I'm interviewing for general surgery residency slots, and will likely follow that with a trauma fellowship.
So what made me change my mind? First and foremost, I realized that I don't want a career that keeps me out of the OR. Second, when I was thinking about what being an ER physician at a large hosptial would be like, I was thinking mostly of trauma, with a few heart attacks and strokes thrown in. My school has a very active emergency medicine interest group and through my participation in that I learned all the drawbacks of that specialty. Bottom line is I don't have the patience to deal with those. Most notably, most of an ER docs time is spent dealing with minor problems that really aren't emergencies. Patients will come to the ER at 2am on Sat morning with a cough, sore throat, pain in the knee, or some other thing that has been going on for weeks and then they become irate because they have to wait while you deal with the chest pain patients. There is a huge debate among insurance companies (who don't want to pay for an ER visit for a non emergency, and rightly so) vs hospitals, who want to be reimbursed for everything, contending that the lay person can't tell what is and is not an emergency (I say that any idiot should know that a cough of 6 weeks duration is NOT and emergency). Plus ER docs are on a salary, often employees of the hosptial, and can be dismissed if too many patients complain about you (and the complaint could be bogus like you didn't apologize profusely enough to the patient who comes in with the 6 week old cough at 2 am and had to wait 3 hours)
That plus the fact that the trauma surgeon is in charge of the trauma in the trauma bay, as well as the fact that you take them to the OR and fix the problem and manage non operative trauma in the ICU, put me into the trauma camp. It also worked out that I like other kinds of surgery as well. And I like the ICU. So if you are thinking that you may do trauma surgery, realize that much of trauma is non operative, and becoming increasingly more so. Also, the surgey board is looking at changing the ways to shorten the duration of training by going to a 4+2 program..4 years of general surgery and 2 years of some specialty. In the case of trauma, it may turn out to be trauma and emergency surgery and would include the appys, perforated colons, ulcers, etc. This is NOT definite yet and is a considerable source of debate in surgery circles (as well as interesting conversation during interviews)
If you are trying to decide between the two, talk to as many folks in both fields as you can. Also ask yourself the question, "How would I feel if I never got to go to the OR again?" after you do your surgery rotation. That should help you decide.