Just my two cents on the issue....
- it is not necessarily true that Categoricals are favored over prelims when it comes to choice rotations or OR time, especially if the Prelim has a surgical career in mind. However, for someone like yourself who has plans on going into Anesthesia, you may be passed over on certain cases for the Categoricals, Designated Prelims and Non-Designated Prelims who want to go into surgery. This likely isn't a problem for you.
- A career in "Surgical Critical Care" which you described as a goal of yours requires at least 3 years of general surgery training, even those without Trauma fellowships attached to the Critical Care portion. More likely, what you want is a post-Anesthesia Critical Care fellowship.
- I agree with SLUser - a Prelim surgical year is pretty painful. While I agree that Medicine rounds are torture, surgery is painful not only because of the hours (which are violated in many programs) but because of the (near) daily humiliation. Many surgeons are unable to criticize and teach without degrading their junior residents. Its one thing to tolerate this because you have a goal of being a surgeon, its quite another because you think it will be better than a Prelim IM year. And I say this coming from a program which is supposedly less malignant than others.
- You will learn Peri-operative care on Medicine, as you will likely be doing some critical care rotations or maybe can even swing a surgical one. Since the vast majority of your colleagues have had a Prelim IM year, it doesn't seem to be a detriment to their career goals.
- If you are still unswayed, yes - there are lots of open spots in Prelim Surgery each year because only those that absolutely have to have them (ie, Ortho, ENT, etc.) apply for them...usually.