For me it's a no brainer - That's because I'm a grammar school snob, primarily because I went to a dreadful comp school.
Some things I think you should consider;
One's mixed and one's boys only - what preference do you and your son have?
Think about the other facilities on offer - sports facilities, after school clubs - which suit your child better?
Think about the average academic range of children in any one year - all HGS students have sat a test to get in. Personally this was a massive issue for me at school - we had four classes ranging from motivated children, to kids I would hate to think where they have ended up!
I know that both schools are focused on improving results and reputation, so that's a positive for both.
Personally I think your son will also stand a better chance of getting into better universities having gone to HGS - but I'm pretty sure some people will try to disagree with me. In reality, I can't imagine you'll get a truly unbiased answer on this forum. Good luck in whatever you choose.