when i first graduated, these were the two areas i was debating between as well.
ER: I did one shift in the ER and although I loved the variety and loved the somewhat "organized chaos", I felt like we were just like a holding area until they moved the patient to a floor or sent them home. I also didn't want to deal with the drug seekers, etc.
PEDS: Love kids, but at the same time I was scared to specialize, especially right out of school. I felt if I specialized, maybe I would lose all my "adult" knowledge since kids are so different. Whereas if you were in the ER you would treat kids and adults.
In the end I chose PICU b/c I was interested in critical care, and pediatrics. Overall it just seemed to be a better fit for me *and I LOVE IT!!!*
Just remember you can always switch again! If you try peds and hate it, then you can always go to the ER later, or vice versa.