I've spent some time shadowing both and while both will offer great experiences, I'd probably go with general surgery. In the ER things can be kind of sporadic. I remember long periods of the doc reading the newspaper or other books; making me fetch stuff like gauze and blood culture kits to keep busy. I've found that in a lot of ER's the Doc just signs off on procedures; very rarely will you get the stabbing victim or the MI, but when cases like that do occur it can be amazing to see what they can do. Now, as far as general surgeons are concerned you really get to see everything across the medical spectrum. You get to experience the in-patient and out-patient care of a host of cases, consult with people in the office, go into the OR, and make post-op rounds. Only thing is, I had to be at the hospital by 5:30am every day I shadowed. Good luck, you'll really enjoy it!