First and foremost well done to your DS for getting offers from two super schools. You should be very proud of him! (And pleased that unless you want to move him the exam stress is over!)
My two DCs are at Highgate. They are happy and thriving but UCS is also a super school by all accounts and if I lived on the other side of the Spaniards' would have cheerfully sent a son there. You can't lose really!
It is so interesting to see what others think of the two schools! I think the sketches of the boys is by and large fair. UCS boys seem to me to be more sophisticated and worldly-wise than Highgate, and Highgate is very sporty, even among the girls. Also, think about the denizens of Islington/Finchley/Crouch End/Highgate/Muswell Hill vs. St John's Wood/HGS/Hampstead and you get an idea too!
Crunch's comments ring pretty true, but Mr Pettitt is a book not to be judged by the cover. He has a clear idea about the appropriate role of a head, and he knows what he believes in. He really impressed me early on when talking about the challenges of raising kids in London. He has said very clearly that he sees it as part of his job to do everything he can to make sure our kids have the chance to be kids and don't grow up too fast. I thought that was marvellous.
Highgate is an older, more established public school, and perhaps the discipline is as you'd expect springing from that tradition. It also has a Christian ethos, lightly applied (the chaplain is a lovely man, and I say this as an agnostic), where I gather UCS has a more modern and secularist approach.
I'd be wary of giving you advice one way or the other. Most of our kids are happy, and we are no more informed about other schools. Sorry if that sounds like a cop-out. I also don't know your family, and what you value. You've seen both schools -- what does your gut say? Trust your instincts because you are the Mummy and you do know best, deep down!
Gypsy is right to stress practical issues like travel because they sometimes get overlooked and school travel is such a big mess up here in NorfLondon. Way back when, starting in the PrePrep, the school run, and later their commute, was a big issue for me and it's not any easier now. About the same time the other branches went co-ed and that is also a big, big plus for us.
Get someone to cover for you and do the runs, and try and park, at the relevant time in the morning. You'll be doing it a lot for a few years, and if one is much worse then keep that in mind. Getting stressed on the school run for five or six years is not good for your relationship with DS!
Then relax because I also agree with TalksNonsense. There is no right or wrong choice They are both good, just different.
Good Luck and let us know what you decide