Hi Lauren,
I think that we all want to do what is best for our children. Moving is a big change under normal circumstances, and especially so when it's an international move. There are a lot of things to consider.
First, it's your child. Every child is different and no system or school will suit every child. As a parent, you know your child better than anybody, so trust your instinct.
Just as some schools in the UK are better than others, that is also true in the US. Different school districts have different criteria for early enrollment, grade skipping, etc. Some schools offer accelerated programs, special education, full day vs. half day kindergarten, sports, etc. I would encourage you to talk to a school about any concerns you have about your daughter before moving into a school district. Make sure that their needs can be met, whatever they are.
What class size is your child used to? Is your child outgoing or shy? Does your child have special needs?
The systems in the two countries differ. That's not to say that I think one is better or worse than the other. Most kids adapt very well, but for some children, the differences in these systems could be more significant than for others.
Also, talk to your daughter's teacher in the UK when it comes closer to the time of your move. He/she should be able to discuss all of her needs with you. My son's teacher was invaluable this way.
I hope it doesn't sound like I'm "over analyzing" but I did have specific concerns for my son and what the impact the change would have on him. Lauren, your daughter sounds very bright and I suspect that is where your concern is coming from and why you are thinking of it now. If you get as much information as possible and follow your instinct, you'll be doing what's best for your daughter. Good luck!