We made the same decision a year ago. (Son's birthday is late August.) At the time, everyone we spoke with (teachers, parents) said in their experience, no one had ever regretted waiting. I remember standing at preschool pickup in the early spring, watching his 5-1/2 year old classmates, and thinking "Yeah, I can see them going to kindergarten." And then I looked at my 4-1/2 year old, and the answer was pretty easy. We had his IQ tested last year, and he's quite extraordinarily gifted, he just was not socially mature enough to handle all-day. His preschool teachers also strongly recommended that we wait. We were lucky enough that his preschool also had a half-day, fully accredited kindergarten that was geared for "young fives" - kids who barely made the cutoff, or just missed it. About half his class went straight on to first grade this year, the other half is now in full-day kindergarten. It was the best of both worlds, really: he would have been a little bored in another year of preschool, so he got to learn what he was ready for (science, math, reading) and still spend the mornings with me and his brother. I know several families who sent their young five to the half-day public kindergarten, and then to the full-day private kindergarten the following year (but that will depend a lot on your daughter, and how she adjusts to major changes - two new schools in two years could be a lot for some kids.) Listen to her preschool teachers - they are going to be able to give you the best information about what she's ready for -her behavior at school is likely to be a little different from her behavior at home. I also thought about things like: "Do I want to send a 17-year-old to college, or an almost 19-year-old?" And "he's got his whole life to be a big kid, why not let him have one more year of early childhood." So we decide to hold him back (and yes, that's how we think of it, we held him back a year.) And it was one of the best parenting decisions we've ever made. Watching him grow last year, and mature, and watching him walk into kindergarten with confidence - he is having a fun, engaged, successful kindergarten year.
I think the suggestion of securing yourself a spot in both places (a half-day program and a full-day program) is really wise - then you can wait and see what happens over the summer, and see if she's ready then. We waited until late spring to make our choice - our son needs preparation time, a last-minute choice would have been very hard for him - but it's true your daughter is going to grow and change a lot between now and then.
I'd actually also suggest you talk to your daughter. Is she excited about going to kindergarten? (Although it's a long, long time in the future for her.) Does the idea of a new school, and being gone all day, make her worried? And to what degree? You can be honest with her about the decision you're making, and that you want to find the place where she will be happy and best able to learn. Especially since she's watched two older siblings go through this process, she may have something valuable to add to the decision-making.
I also want to say: I know there's so much pressure to "make a decision right now!!!!" But you can always change your mind. It's just kindergarten. If you start her, and it's a disaster, you can pull her out. If you keep her in preschool, and it's a disaster, you can move her up. Or if you keep her in preschool, and it's a good year, but by the time she's in kindergarten, she's ready for first grade, a good teacher will know to move her up a grade. People move across the country in the middle of the school year. Kids change schools. Or classrooms. So give yourself (and your daughter) a break, and know that you are not determining her life destiny in making this choice. Not that the choice is irrelevant, but it's also not destiny.
Good luck - you will find the right place for her.