I don't think that there is such a difference in the ability between boys starting at Wilson's and those starting Wallington
. I certainly wouldn't take any account of what a relatively small number of parents seem to prefer on a forum. There are lots of people who just seem to focus on a league table results without realising that there is separate entry criteria for 6th form, so looking at A level results is pretty irrelevant - your son may not get the chance to take them at this school, and the order in the league table may change over time especially now that Wilson's has increased to 180.
There is a difference in ethos though, so it is worth thinking about where your child will thrive. Some love the strict no-nonsense approach at Wilson's where you do what you're told without question. Others prefer the slightly more relaxed and family focused approach at WCGS. It isn't that much different when looking at schools such as Whitgift and Trinity. Whitgift is stricter, Trinity more relaxed. Both produce excellent results. Some of my friends prefer one, others the other. There is no point in putting Wilson's first choice and then complaining when your son isn't allowed a smartphone, or gets a detention because his shirt is hanging out. Nor should you choose WCGS and be upset about making GCSE options in year 8.
What is noticeable on the forum is a lack of parents regretting going for these schools.
My eldest son only sat for WCGS, as they did rugby when Wilson's didn't.