Define "saving a life". Do you want acute, the patient is going to die in mere minutes or seconds without intervention, or more if something hadn't been done death was inevitable.
There are very few positions where someone will die in seconds to minutes unless you do something. Really, probably only trauma surgery offers on a semi-regular basis. Emergency medicine certainly has the potential, but you also have to deal with a lot of non-critical illnesses and acting like a primary care doctor.
Critical care (either pediatrics or adult) deals with really, really sick patients, and will get the lucky souls who actually made it out from under the trauma surgeon's knife or away from the ER. They may come in stable and then crash, requiring lots of interventions and things like pressor support, vent management and ECMO. (Of course, the way higher survival rate in the Peds ICU makes it a happier place in general)
Likewise, cardiologists in both the peds and adult world can have a direct impact, but this would have to be a conscious career decision by these individuals to put themselves in those sorts of situations.
Of course fields like Oncology, the progression towards death is much slower, and thus how you define saving a life really changes things and if you view that as "saving a life"...