The kindergarten decision was the hardest parenting decision we have made thus far. I totally understand.
DD's birthday is in Nov and the year before she was set to go to kindergarten, she was in preschool. When we asked her about kindergarten, she said "kindergarten, why would I want to go to kindergarten?" I put her in preschool for another year and at the end of that year, she was saying "yay kindergarten. I love kindergarten!"
It was the best decision we ever made. She's now 15 and excels academically AND socially and she has been through 4 school systems in three countries and has been to 5 schools in the last 5 years.
The advice we got when we were making the decision about when to put her in kindergarten, came from a friend of mine who was doing her phD in ed psych at the time. Her professors said, if you have the slightest doubt, keep her back, because it will give her a social edge which will give her the confidence to carry her through. They were absolutely right. Better to have a kid who is over-confident than a kid who is always struggling to keep up.
Also, the social gap doesn't necessarily show up in the early primary years but it will show up in Jr. high. Another friend of mine is a jr. high teacher and she thought we were making a huge mistake, keeping DD back. She was so upset with us that she went to talk to her school counselor who simply said to her "who are the youngest kids in your classes?" My friend could answer immediately and funny, all of them had the late birthdays and almost all of them were struggling socially.
I think also, you'll find that kindergarten is very different in the states, depending on where you'll be living, of course. In San Diego, for instance, it is all-day and the kids have a placement test and are placed according to ability. And you can now fail kindergarten. I know that if my son knew how to read, he would have been given tons to do. The programme was designed for every different level. But for him, his biggest challenge was just getting through the day.