If I had a local option I would go for it if it was ambitious enough academically. It sounds as though you need to stand at the gates of Trinity and interview year 8 parents of top set children and ask to view their exercise books from last year.
I don't think that Knole can offer the range of academic subjects one might hope for in the sixth form, but then there is a lot of sixth form swapping.
I think sixth form options at both Knole and Trinity are likely to be limited, except in co-operation with other schools, the problem with Trinity being its small size (4FE). However, there is also the option of starting at one or the other and then applying to transfer to a different school for the sixth form (Weald of Kent?), subject to successful GCSEs.
It is so difficult getting a handle on the relative merits of Trinity and the Knole grammar stream, both being so new.
Trinity's recent OFSTED report was less good than I expected, but accommodation issues will have placed additional demands on staff and students and I think it will be difficult to get a reliable picture before they are settled into their new, permanent building 2016-17.
Knole's GCSE results this year should have been better, I feel, if there has been a drive to raise overall standards since the amalgamation of Bradbourne and Wildernesse. Presumably too, some of the grammar stream teachers have also been involved in teaching outside of the grammar stream, so their successes and failures are reflected in the school's GCSE results.
If anyone can offer anything evidential or even hearsay on any of my speculative points, that would be much appreciated!