Although Nonsuch does seem to do better than Wallington in the league tables, I wouldn't put too much emphasis on this - bear in mind that in the great scheme of things they both do fantastically well
I would consider other things such as the fact that Wallington has an engineering specialism whereas Nonsuch has science and languages specialisms. Will this affect the choice of gcses (eg at Nonsuch all girls are required to take 3 sciences plus one MFL).
Are you happy with the facilities at Wallington compared to Nonsuch e.g. sports facilities at nonsuch are very good, drama maybe not so good. Music is very good at nonsuch and parents have recently been asked for their views on how to make it better.
Don't underestimate the difference that travelling can make either, though I don't think a 40 min bus journey is overly difficult, particularly as the services are very frequent in that neck of the woods.
To sum up they are both fantastic schools and don't be swayed by a few places in the league tables. (My DDs are at Nonsuch and love it by the way.)