Interesting post.
We only have one child although this was not because of any decisions regarding schooling, just the way it ended up. Ds has had a state primary school education, which has been fine. He is now nearly 11 and we are considering secondary options. We are considering a private school for him, and one of the main reasons we are able to do so is the fact he is an only child, but we did not plan it this way.
In terms of siblings, I have 2 sisters, get on reasonably well, don't live close but keep in touch by phone/e-mail regularly. Dh also has 2 siblings, rarely speaks to them/contacts them. They are not on particularly bad terms, just don't have much in common.
Ds has never yearned for a sibling, and is quite content being an only. I don't know whether this is good or bad.