I enjoyed both rotations as a med student and considered both as a career choice. Both EM and IM often involve working up undifferentiated patients and coming up with differential diagnoses so I don't think its unexpected to like both fields.
EM Pros:
- In many ways, your job will be that of the ultimate generalist. You will learn to manage pregnant women, kids and trauma in addition to adult medical problems
- Lots of minor procedures like lacs, I&D, etc that you don't do quite as frequently in IM
- When you're done with your shift, you are truly done (no labs to follow up on, patients to contact, etc)
EM Cons:
- Frequent night shifts
- As Reddpoint alluded to, the ER is not the place to get to the bottom of a complex diagnosis. You will probably never realize the routine community acquired pneumonia you admitted actually had IPF, or the abdominal pain actually had hypercalcemia due to MEN1.
- No long-term patient relationships (I really underestimated how important this was as a medical student and even as an intern)
IM Pros:
- Numerous opportunities to specialize and tailor your job as you see fit. You can do shift work as a hospitalist, work 8-5 in a clinic as a PCP or rheumatologist, focus on advanced procedures (therapeutic endoscopy, interventional cards/pulm)... the list goes on (can even run an ER at a VA).
- Longer-term patient relationships
- Opportunity to get to the botton of complex cases and learn how to manage patients who haven't responded to the standard stuff in pocket medicine/up to date
- Far more structured opportunities and mentorship if you have any interest in a research-based career (although EM departments do conduct research, it is typically nowhere close to what IM departments have to offer)
IM Cons:
- Hospitalist: lots of paperwork, social issues. In practice (out of training) procedures often handed off to IR since they even a simple LP can take an hour and ruin your efficiency
- GI/Cards: often have to come in at night to scope a variceal bleeder or cath a STEMI or at least field calls and troubleshoot
- Any office based specialty: need to follow up on tests, share call (even if its home call in a field like oncology or endocrine or outpt primary care)
Income: this is actually difficult to compare because for one, IM has many subspecialties with very disparate pay. Also, the pay differs between IM subspecialities, between private practice vs academic (and type of academic), and region.
EM/IM:
There are 5-year programs that train you to do both, and I have met attendings who split their time between the wards and the ER. The flip side is the folks who trained in this combined track and now only practice in one field have essentially wasted 2 years of training/income.