I am a staunch supporter of the state system, as I believe that the best way to prepare one's child for modern society, with its many ethnic and eco-socionomic backgrounds, is to send them to a state school. However, it all depends on what your boy is like and what you want to get out of the school.
WCGS will definitely help your child fulfill his potential. They certainly did mine!
Whilst pushing the boys quite hard during lessons (as you would expect), there is plenty of space for fun and games. The house competitions such as House Music, House Drama, House Arts, are all designed for the creativity of the children to flow freely, and they can be pretty silly! Not everything is high-brow Culture.
When choosing WCGS originally, what clinched the deal was that someone told us that they are a bit eccentric, and so is my eldest son, so they would go well together.
Discipline is important, they aim to produce "independent learners", care a great deal for the individuals, are very approachable (emails get answered promptly), albeit sometimes a tad disorganised.
What you need to consider if you are weighing it up against the indie, is money. Money is tight, whereas, obviously, that would not be a problem in and independent.
How much does your child want to do sports etc as a part of the curriculum? WCGS does lots of sports, but obviously a lot of it is cocurricular. In the indie they will probably timetable a lot of it.
As your boy must be bright, he will do as well academically in the indie as at Wallington.
As far as the single sex issue, I was also concerned, but it does not seem to have been a problem. Okay, it might be less likely that they have a girlfriend at school before the age of 16 (when the girls arrive in the sixth form), but hey, that means less distraction!
The WCGS Federation (New academy)... My guess is as good as the next one's. They assure us that none of the current staff will be syphoned off to go to the new school. I hope that is true. The Federation will make sense economically, and that is important in the next few years, plus that may even be a deciding factor for you.
I hope I have been of some help here. We know the school warts and all, and, having looked at all the other grammars, have decided to send no 2 there again.