Why choose one field over the other? Because you find one more interesting and can't imagine doing anything else. They are very different fields so explore both until you find what is important to you in a career. The things that you mentioned are important to you, but realize that in the long run they don't matter that much. What matters is what you like and what you see yourself doing for the next 30 years. Both fields are becoming highly specialized. Think of where you want to be in 10 years. Now does an anesthesiologist or a surgeon fit that role? Third year is a tough time to decide on a specialty. You should really do a rotation in each, as I'm sure you have at least done surgery. Anesthesiology looks a lot different from the other side of the curtain, however, so if your only experience is watching what they do on a surgery rotation do a rotation in anesthesia. Also realize that outside of an academic med center both specialties are different. Private practice surgeons and anesthesiologists lives are as a whole better in terms of income and lifestyle than their academic counterparts.
(considering life style, future of the profession, being intellectually challenged, salary, length of residency, etc.....).
Lifestyle-both are 24 hour specialties and this isn't going to go away. You can find jobs that have more or less call, but in either you will take call. It really is dependent on your group and where you practice. Surgeons do usually spend at least 2 days per week in the office though. They also take care of their patients post operatively. This means rounding.
Future- Both have a bright future. Both are on the upswing in terms of quality of applicants/residents (surgery took a dip a couple of years ago but it's back with the 80 hour rule, anesthesia had a few pretty bad years in the mid-late 90's). Good people in a specialty ensure a good future. New technologies continue in both fields, interventional pain is pretty new and there is some fun stuff there. People are always going to need general surgeons, diseases just don't go away. I've talked about CRNA's and the future before. In short, don't let this sway you away from anesthesia. Nearly every specialty has some competition from mid-level providers.
Intellectual challenge - Both offer this, but both have their bread and butter as well. General surgeons will always do choles, hernias, breast ca, etc. After some time I'm sure that these become routine just as the anesthesia does for them. There are opportunities in both for as much challenge as you want. Cardiothoracic and peds in both specialties is challenging.
Salary - Anesthesia is booming right now. How long this will last is anyone's guess but it's likely to be some time. From what I have seen, starting salaries for anesthesia are higher than general surgery at the present time. I wouldn't let salary guide your choice, however. Both specialties will give you more money than most other specialties with the chance to become partner in very lucrative practices.
Residency length- 4 vs. 5 years. Again, don't let this guide your decision too much. If you wanted, say neurosurg then maybe it would be an issue, but one year won't seem that bad in the long run. Especially if you spend all 4 years in anesthesia looking over the curtain thinking 'that could be me', they will be a very long 4 years.
Now hopefully this doesn't dissuade you from anesthesia. I just want to make you think about what makes you happy. Here are my reasons for anesthesiology:
I love acute care medicine! There is no other specialty that allows you to use all of your pharm and physiology knowledge like anesthesia. Things happen quickly. The patient interaction is intense. It is a diverse field, even for the generalist, in respect to sites of anesthesia (psych hospitals, MRI, cath lab, OR, ICU, L&D) and diversity of patients.
In closing, my advise is to first do a rotation in each. Then find what you like and what will keep you happy for your career. Keep asking questions. Shadow some private practice docs in both specialties.
