There are so many factors at play, however, I agree that there are no right answer. In addition to disposition and birth order, there is the question of the school environment - how supportive, how challenging, and how structured it is. 

I redshirted my boy - sort of. His bday is 10/29 and our deadline is 12/1, so he could have gone ahead in this state, but not in some others. Although he is ahead of his peers academically, he is not bored. In addition to the academic subjects, he is learning about playground politics, sports, magic, and graffiti (terrific!) He is a mild-mannered little dude and I think his confidence in matters of the classroom has been great for his self-esteem. I lost much sleep over this decision, but was ultimately guided by the notion that no one ever regrets holding their kid back, though some may regret pushing them ahead.