Dear K.,
I feel for you. This is a very tough decision all around.
I had to make the same decision. My daughter would have turned 5 two weeks after kindergarten started. Yes, she "made" the cut off date, but we decided not to send her that year. I was having a very hard time sending my 4 year old off to school. We also had another child 13 months younger that missed the cut off date. I decided to wait so that they would be back-back in school b/c they are only one year apart. Also, I thought about all the things you are thinking about. I wasn't concerned at all about the academics of early elementary years but rather, was concerned about her being the last one to drive a car, etc.
Also, if I had sent my daughter right before she turned 5, she would have missed the age deadline for soccer, softball, etc and would have had to play with girls a grade younger. I decided it made sense for her to be able to be in sports with her classmates and waited that year.
Fast forward a bit, my daughter is now 13 and in 7th grade. As hard as it is to admit, I think overall, it would have been much better if I had sent her earlier. She has always been a bit mature for her grade and sometimes it's harder "fitting in" with these kids. (She does better with the kids a grade above her.) Also, academically, she is VERY ready to move ahead. I am also a teacher and we have supplemented at home. Overall, our daughter does very well in school, but socially and academically would have benefitted from being in the grade above hers.
By all means, if there are any "red flags", then I would wait. (social or academic reasons) However, if academically and socially she is on target (or above) I guess I would seriously consider sending her. In my experience, waiting didn't give her "an edge" but seemed to cause some frustration. I have no doubt that my daughter would be perfectly fine academically and socially if she were a grade above. (In fact, it would probably be a much better fit.) (I also have a child in high school and one in college, so I know the pressures/stress she'll be facing, but I still think she'd be fine.)
Good luck with this decision.