I am in a somewhat similar position as you. Since DS7 has been in public school for 2.5 years - a school that has been kind of nightmare to deal with - my general opinion supports keeping my kids out of public school as much as I can. Right now, DD3 would easily be ready for K next year. But I would rather keep her in a (great) local preschool part time next year and have her enter K at the regular time for her, when she turns 5. I will then work to accelerate her out of K when she gets to school.
For my DD3, repeating K would be a nightmare - I can see it already (she is working on addition, subtraction now and playing with the idea of multiplication). I don't know what your preK is like, but the preK that DD will go to will happily accommodate her academic needs as they did with DS7.
