I would echo the question asked by PonyBoy. ER physicians, with respect to trauma, are mostly invovled in the initial evaluation and stabilization of trauma victims. They do not participate in any operative interventions that the patient may need. However, the truama surgeon (usualy a general surgeon who has specialized training in trauma) is inlvovled in the care of criticaly injured patients from the time of arrival at the emergency department until patient discharge. This includes assisting with initial stabilization, interpreting lab and radiographic studies, operating if needed, coordinating the care of the patient from the intensive care unit until discharge and beyond. So, the ER physician has a more limited role in the care of the severly injured trauma patient than the trauma surgeon.
Because trauma surgeons must be general surgeons first, one needs to consider this when applying to medical school. At this time, trauma fellowships after general surgery training are moderatley competitive. However, competition is increasing from year to year. As the competition increases training programs can take the pick of the litter (so to speak) of applicants. This usually means one must train at a University based general surgery training program and particiapte in one-two years of research during residency. Generally, MD's have an easier time getting into university based general surgery programs than DO's, but this, of course, is a generalization and there are other factors involved.
I hope this helps.