A bit of a bias towards Nonsuch here, as a good friend of mine went there and she turned out very well, as they say - but that was decades ago and the school might well have changed a bit since then
What did you both / all think when you went round the schools? Presumably you actually ranked Nonsuch above Newstead Wood when you filled in your CAF but was this a question of really having a strong preference for Nonsuch over NW, or did you actually rate them pretty much equally but obviously had to put one first? Its a pity that its the holidays now - I assume that you will have to let your LA know your response well before there is a chance to ask to look round both again.
As DC17C says, the practicality of the school journey is also something to consider. Another thought - voiced on a thread elsewhere on the forum - is to say to yourself and DD, 'The place at Newstead Wood / The place at Nonsuch has been withdrawn' - and see which statement upsets you / her the most.
They are both good schools, though, so please don't get hung up on league table rankings as a reason to choose....