My son is a November baby. He could have started kindergarten this current school year, but we chose to hold him back for a variety of reasons. The first was maturity. He has had some social issues in the past, but his preschool teachers assure me that he is doing great now. When I spoke with them about the possibility of keeping him one more year in preschool, I thought the teacher made a good point. He would probably be fine NOW, but it's in 8th grade or so, when puberty sets in, that you do tend to notice a bigger disparity in both physical and emotional maturity.
Which leads me to the second reason we held him. My son is short. I thought it would be helpful to give him another year to try to catch up with the kids who will be in his grade. We actually attended a birthday party of kindergarteners this morning, and he was by far the littlest. Again, it's probably ok now, but as he gets older, I think it will be better for his self-esteem if he is not the shortest child in the class.
Finally, his sister is 3.5 years younger than him. I like the idea of having them be three grades apart, instead of four. That means they will have three years together in elementary school and one year together in high school (instead of two and zero, respectively), which I think would be more fun for them and more convenient logistically for our family.
The one issue that I am concerned about is that he will most likely be more academically advanced than the other children in his kindergarten class, partly because he has had an extra year to work on his letters, numbers, etc. But I suspect that probably would have been the case regardless, so I'll just try to keep an eye on that next year.
Good luck with whatever you decide. You still have another year to think about it. Talk with his preschool teachers and see how you are all feeling about it as the time gets closer. Personally, I'm so glad I have had this extra year with my boy.