K.B.
I went to Public and Private school growing up. I can tell you from my own experience that I actually enjoyed the Public school more when I got up into high school and probably would've done fine at the public middle school as opposed to the private middle school. I also did public school for my early years. I was still able to get into every college I applied to, had great AP opportunities and extra curriculars. Personally, I think that unless your in an area where you know the public schools are terrible, I would find another way to use that money (also keep in mind that college costs are increasing in alarming amounts every year. The 4 years I was in undergrad, my tuition increased 130% by the time I graduated in 2008. This is not even talking about if she ends up wanting to go to a more prestigious private university, where tuition is already close to 30,000 a year, or grad school which can add another 2-3 years depending on what she wants to do. I don't know how much you have, but it may be better to save as much of that as you can, since you don't know what costs will be like 15 years from now). If the public schools are good, I would stick with them. You are right that the most important part is you and your spouses role in her education. If you focus on education, support her, advocate for what she may or may not be receiving, etc., then you should be alright. The smaller class sizes can be beneficial, but I found when I switched around (as well as my mom found), that there really wasn't THAT much of a difference in class sizes with the private and public schools in my area.
Also, I would take this information on private school kids testing better then public school kids with a grain of salt. Private schools select which kids they want to have come to their school, usually through initial testing and an interview. Public schools do not have this option and have to enroll anyone who lives in the district. Private schools can also dismiss kids who do not seem to be taking their education seriously, where as public schools can't do that unless there is behavior problems (and even then the kids just enroll into another public school). So the comparison groups are really not similar to start with because private schools will admit the "higher achieving" kids and reject the "lower achieving" kids where as public schools have to take BOTH. (This is not to say though that there are not problem kids in private schools. There are plenty. It's just they usually will test better in general because that is how they got into the school in the first place, so that can't be compared fairly).