The entire thought process is totally different. EM folks are "treat 'em and street 'em". They don't really care about the pathophysiology, WHY things are going wrong, or how long to prescribe antibiotics for this nasty osteomyelitis or what bugs are potentially there. They know empiric treatments for the most part and don't care beyond "what is the first dose of antibiotics". They don't care about sensitivities to drugs, side effects of long term treatment, or maximizing patient's functional capacity. They care about "what do I need to do RIGHT NOW to get this person stable enough for the floor or out the door"?
IM folks think about longer term. They treat empirically and then trim after sensitivities come back. They consider functional capacity. If you put someone on IV only antibiotics, how do you manage that? hospital? nursing home? PICC line? infusion center? what about long term management of chemo related diarrhea? how about the effects of metformin on renal function? considering xigris for sepsis? vent settings based on ABGs? exactly WHY does this 32 year old person have cardiomyopathy anyway and how should that best be handled???????
Basically, EM fixes the immediate threats and then turfs to IM. EM doesn't care WHY, they just fix the NOW. IM finds out WHY and deals with any NOW things that come up along the way (such as hemorrhagic anemia, pancreatitis, wound dehiscence caused by odd bacteria, renal failure needing dialysis, cardiac dysrhythmias caused by electrolyte disturbances secondary to high ostomy output, etc.). IM folks wonder what happens to their patients afterwards. EM folks see a repeat customer and say "you AGAIN? NOW what?"
If you are a WHY personality, then IM is a better fit. If you are a NOW personality, then EM is a better fit.
Now having said that, the programs look for those things. Are you a decision maker or a puzzle builder? Can you make rapid decisions (and make the right ones) or do you need more time? Do you like procedures or not? do you think about your patients when you go home or not? can you swing your shifts on a moment's notice? Can you juggle 12 patients at one time or do you want to think about only 6? Do you thrive on chaos and sorting out the mess or do you like it neat and tidy? Programs look for that "fit" of a personality into the specialty more than anything else.