I know this can be a difficult decision, but let me throw in my $.02.
My husband and I were both classified "gifted" when we were kids. My birthday also made me borderline with regard to which grade I should be in. I was "held back" at my mother's request in spite of my advanced reading and math skills (she was a teacher by training) and was one of the oldest in my class. I had the classic "boredom" issues and was in trouble a lot the first few years, but then I was able to move into a gifted/talented program and I fluorished.
Meanwhile, my husband skipped first grade at the recommendation of his teachers/school because he already knew the material. He did fine in second grade and through elementary school, but being younger than his classmates caught up with him socially in middle school to the point that he actually became suicidal. It can be brutal if you're the only boy (or girl, for that matter) that still hasn't started the changes of puberty. Imagine a short, skinny boy with glasses in a locker room with boys who are all starting to get broad shoulders and to shoot up in height and to develop in other ways.
So even though both situations were difficult, I would take the boredom issues (which were resolvable) over the social issues in middle school (which were much more difficult to resolve) any day. Our son has a September birthday, which here automatically makes him the oldest in his class, but in North Carolina (where we come from) would have potentially made him the youngest, since their cutoff is October 15. We would NOT have wanted him to be the youngest in the class, based on my husband's experience.
To sort of confirm this, I had a conversation recently with the administrator of my children's preschool on this topic. I made the remark that I felt that maybe 10% of summer birthday children were really ready in every way to go into kindergarten. Her answer was that probably 50% were ready for kindergarten, but that it would catch up with them socially in middle school, which is why she always recommends holding kids with summer birthdays back. I thought, WOW! That's EXACTLY what happened to my husband! But how wonderful to hear that kind of far-thinking wisdom from a preschool administrator!
Anyway, you now have my $.02.
B.