Pedi Cardiology: Seems to me to be a "cool" specialty with lots of procedures and options for further sub-subspecialization. For example, you can do electrophysiology, interventional caths, or critical care cardiology (but that means more training probably). Or you can just open up a primarily outpatient office. Or you can be a pseudo-radiologist and read echos for a nice lifestyle.
Neuro: Fewer procedures, more "thinking," but still looking at a lot of images and maybe some things like NCV studies or mapping for epilepsy surgery with the neurosurgeons. One nice advantage of pedi neuro is that you can do a 5-yr combined residency/fellowship out of med school and not have to apply again and not have to spend 6 years. If you want. But you have do do some adult training in there.
Are you at a point where you need to decide these things yet?
They seem to have pretty different personalities, so I think doing a rotation in each would probably show most people where they fit or don't fit.