Whitgift have formalised their "BTech pathway" now for 6th form, so there should be no surprises for parents along the way, and fewer points will be required from those boys only sitting 8 GCSEs to get into 6th form now.
As far as I can see, if you looked at the top 100 pupils at each school, you would see similar, superb results, probably ahead of the grammar schools bearing in mind some of the boys wouldn't have got grammar places at 11. One of the differences is how long "the tail" is, and how it is managed. I think it is difficult for any school when it appears that a lot of parental decisions are made by reference to the league tables, in particular by the percentage A-A*% at GCSE and A level. But the value added for these schools is probably fairly high, though I haven't seen any hard data to support that.
I think that the difficulty as a parent, especially of a 10 or 11 year old is that you can't necessarily predict where your child will be on the academic spectrum at 15, and even less can you predict the route by which they will get there! Different schools have a slightly different approach, which is why I think it is important to know as much as you can about the particular ethos and direction of the school, on the basis that if you support it, then you'll be more likely to be happy with future changes (let's face it, do any of us know what sort of qualifications the boys for 10+ will eventually sit for?!).
Parents and boys seem happy at these schools, with the only disgruntled parents that I know being those at Whitgift who either didn't make it into 6th form, or discovered that they weren't going to be entered under the school name for a couple of GCSEs and Whitgift seem have made changes to prevent that from happening again in future. I will be interested to see if Whitgift does anything to improve the interview/callback process this year, as again it is one of the few areas of complaint.
I guess the one common difficulty for both W and T is that they do give out a high amount of subsidy, and that can inevitably lead to some parents feeling as if they are personally subsidising other children. It might be easier to see why you would subsidise a truly worldclass musician or sportsman, and slightly harder to see why Bert should get 20% off because he was good at sums at 10, or even just because his parents decided to ring up and "haggle". The move towards honorary scholarships is an interesting one.
Good luck to all of those heading to Big School shortly. May your wait for a phonecall be a short one!