Being one who has completed a med-peds residency and now training in emergency medicine, I found the job market in most cities is tight for non-EM trained physicians. Many non-EM boarded physicians are not being hired anymore, and as a result, they have limited job opportunities other than their current employer. Indeed, you'll find the med-peds training superb for 90% of the EM patients. However, for the other 10% of the time you're working in EM, you'll find yourself inadequately trained for such situations as airway management and trauma. Due to the lack of training in EM, your susceptibility to malpractice and your malpractice insurance itself are significantly increased over the EM trained physician. Studies have shown that training in EM lowers your malpractice substantially.
After speaking with several chairs in EM, they advised me that a peds EM fellowship training is less marketable than EM training. The number of jobs is less, and the salary for peds EM is substantially less than general EM. A peds EM fellowship is merely 1 year of additional clinical training with 2 years of research as is the case with all pediatric fellowships. By doing an EM residency now, I will gain about 6 months of peds EM training which will provide enough knowledge and experience to care for most sick children.
If you know you want to do EM, do a residency in emergency medicine as it'll shorten your training by 4 years. Clearly, the medicicine and peds training is helpful, but it's not necessary to be an excellent EM physician. For someone such as myself who is pursuing an academic career in adult and pediatric EM, the unusual path in which I took (not intentially) will hopefully prove beneficial.
I'd be happy to speak in more detail about this discussion if anyone has any questions. Have a good week.