You need to bear in mind that not all 11-year-olds at CH leave so you need to look at the 13+ destinations too.
I only have detailed info on CH's results.
For 2008, at Junior Entrance level, 18 boys went to Whitgift and 6 went to Trinity from CH. Of the rest, 4 went to Sutton grammar schools and 1 went to Royal Russell. 2 others went elsewhere and the rest stayed at CH.
Also in 2008, at 13+ level, 6 went to Whitgift and 3 went to Trinity. The rest went to all sorts of schools (including Eton, Westminster and Epsom College).
I know a few boys at Elmhurst and I do think CH push them harder. Some boys move to CH from Elmhurst at 11+ (primarily if they don't get into grammar school or Whitgift/Trinity at that stage). By 11 they are pretty much on a par for Maths & English but CH boys seem to be ahead in French. And as I said previously, Elmhurst don't do Latin whereas CH boys start Latin in Year 5.
Sport is quite big at both schools and both teach swimming from an early stage. I would say music at CH is a notch above Elmhurst, though.
Are you absolutely set on one of these two? Have you looked at any of the other preps around Croydon?
Croydon actually has some really nice primary schools (particularly around Sanderstead) but sport can be a bit hit and miss.
It really depends on what you are looking for, in all honesty.