I am guessing you have not visited any of them? I know both Twickenham Prep and Newland , not intimately. I have visited and considered both and know parents at both.
I loved Newland House - but it is very oversubscribed, so get your child's name on the list now. I moved to the area when my dc was 3 and could not get them in at all, unfortunately, there was an opportunity at 7 but for various other reasons I did not go there. Friends with children there are very happy in deed. Twickenham Prep is also a good school with many happy parents - different to NH in that it appears to be a bit less academic, with a more gentle approach to learning. And a bit more religious which attracts some, repels others. But it does not pretend to be otherwise.