My son was very smart and mature for his age and was also just a few months too young to start our 6 year old kindergarten. He was in an excellent home day care/preschool and we chose to keep him there for one more season and start him in kindergarten at age 6.
I have to say, it worked out wonderfully. If anything, he was just more prepared for kindergarten and it set him up for success and he had a great time there. Remember that just because your daughter COULD go now- why SHOULD she?
If you feel like she is just being babysat in preschool and isn't learning anything, that is something to consider. Is she bored at preschool? Complaining to you or misbehaving or acting out? Are there other kids her age there for her to play with?
Also, consider how socially mature she is. My son was very socially mature, but being in preschool the extra year allowed him to really enjoy being one of the 'bigs' at the school and helping the 'littles'! It gave him a lot of pride and self-confidence to be one of the big kids and taught him to be very patient and kind with younger children.
Kids are just absorbing knowledge EVERYWHERE at that age! I am sure you read to your daughter and do all sorts of things with her at home as well as in preschool. Who is to say she would get more educationally out of kindergarten starting early than she does with you???
As a full time working mom, I TREASURE the few early years I was able to stay home with my son. Don't be so eager to start the school merry-go-round- it will last for years! Enjoy your girl and let her stay little a while longer is my advice!