I was torn between Med-Peds, IM, and general surgery. Ended up doing Med-Peds. It's not for everyone. It's 4 years rather than 3. It can't be compared to FM in that it is predominantly inpatient based, much more unit (6-7 months of CCU, MICU, PICU, NICU) and ward (I don't know how many months) and ed time (6-7 months ED and urgent care) than FM, and it's half IM and half peds (whereas FM has something like 6 months or less of peds altogether). My own perspective is that most Med-Peds people are internists by mind set. They see the analogies between adult and pediatric disease, they are not offended/annoyed by kids or their parents, and they try to apply the rigor of the internist to the pediatric patient, which is not the most natural or even effective way to do it.
There are ways in which pediatrics is easier than internal medicine (disease burden and complexity for one) and ways in which it is much harder (unreliability or absence of subjective history, noncompliance with physical exam maneuvers, fretfulness within the field of alarming parents with serious disease on the differential, the aversion to radiation exposure creating high thresholds for imaging, the need for sedation or anesthesia for routine imaging and procedures, etc.). The anxiety levels are much higher with sick children and especially sick babies than with adults.
The compensation for medical subspecialties is much higher than for pediatric subspecialties, sometimes ridiculously so. Peds hematologists-oncologists get paid $~100K starting out. Adult counterparts get paid $~250-300K. Unfortunately, society and the government do not respect those who treat children with cancer the way they do those who treat adults with cancer.
If you hate taking care of adults and are willing to take the paycut of pediatrics, then do pediatrics. If you enjoy taking care of both, think about Med-Peds or an organ-focused/surgical subspecialty (although, correct me if I'm wrong WS, but I think surgeons are now so subspecialized that the pediatric urologist will not be doing adult urology and vice versa, etc.). FM unfortunately is predominantly outpatient based and does not offer fellowship opportunities. The population they treat is predominantly adult and geriatric (just like outpatient and inpatient internists).
My general impression from your first post is that pediatrics is the right fit for you, you just need to figure out whether or not you will miss adults too much to give up on them entirely and whether you are willing to tolerate the paycut compared to most other fields in medicine.
