I was on the fence for a while too. In the end I chose IM for these reasons:
1) It is extremely important to me to relate to my patients as peers. This is virtually impossible in peds because the decision-maker is that neurotic irrational Munchhauseny freak of a parent and not my patient. I was constantly irritated by the conflict of duty to the patient and the legal need to talk to the parent.
2) I want to do hospitalist medicine. That means sicker patients. It's easy for me to genuinely commit to care, whether resuscitative or palliative, of an adult who has lived a life, made friends, laughed, loved, drank beer, whatever. I could not genuinely commit to a 6 month old with Three Dead French Guys Syndrome who I knew was going to die by 12 months of age after spending their one and only year never knowing anything but pain and suffering.
3) I thought IM diseases were more interesting.
4) IM personalities fit better with me that pediatricians.