In my experience, most of what you will see in the ER is more medical than surgical. Yes, you see lots of fractures, lacerations, etc., but unless you work in a pure trauma center, you are going to be faced with a lot of medical emergencies. An acute MI is a wonderous thing to behold, and can be every bit as emergent as a trauma. Quick recognition of changes and treatment are a must. Subtle things can mean the difference in life or death. If you are in triage a while, you will see what I mean.
Diabetic emergencies must be dealt with quickly. If someone is unresponsive, you don't know if their sugar is 15, or they are a head injury, CVA, or whatever could be the problem, so you need to be able to do quick assessments.
Allergic reactions, anaphylaxis, is a life threatening situation that must be recognized quickly.
Asthma, COPD, CHF, all can lead to acute respiratory distress/arrest, so things must be recognized and treated quickly. Remember your ABC'S? Well this one is the big A.
Being able to recognize "unhealthy" sounds is important, i.e. stridor. Also, you must be able to recognize unhealthy color. A chronic renal patient is generally somewhat "gray", but nothing like a big MI pending arrest.
Based on what I have reviewed above, I think a medical background is more beneficial. In the ER, if you have a trauma coming in, you generally have some notice (EMS) and they have begun the stabilization process, IV, C collar, etc. I know there are those times a GSW is dropped off at the door, but that is not the rule in most hospitals. You also have more support in a trauma center, surgeons, OR available 24/7, etc. which you don't always have in a non trauma or non teaching hospital.
But when you are the triage nurse, and someone walks in not looking too well, and all the beds are full, YOU must be the one to decide if they can wait, or if they need to come back and kick someone else out of a bed. It can be a daunting task sometimes, juggling the sickest of the sick.
Good luck, hope my rambling makes some sense.