Hi London. Congrats to your son on his stunning set of offers!
I'm afraid I have little knowledge of WU (DS didn't apply as there was no way he was going to get one of those 10 places at 11 plus - maybe this tells you all you need to know ?? )!
He is now at LU in Year 7 though. To be honest, I never looked at the number of Oxbridge places the school secure. There does seem to be an increasing number going to US unis though and there is a department at LU to advise on this process.
LU seems to attract very articulate, outgoing types. For instance, they all want to be in the debating team or take lead part in the plays, etc! I think being co-ed gives it a livelier, edgier atmosphere than a single sex school - obviously this would not suits everyone though.
Another consideration I'd that they do not have to work to the Common Entrance syllabus as LU do not really have an intake at 13 plus.
At LU there is no streaming at all until some time into year 8 and no exam week in the summer for Year 7s. They do not give A/B/C etc grades on report cards - there is simply a 'L' grade which means that a pupil is at LU standard, an L* grade if they are consistently top of the class in a particular subject, or an R grade if some areas need working on. It's made very clear to all that a report consisting of L grades means that a DC is well on track for the highest grades at GCSE and the vast majority of pupils will be achieving Ls. So no need to stress as long as you are getting Ls!
The whole school closes for a week in June for 'Activities Week' when they go off on a residential trip of their choice (chosen from a quite substantial brochure)!
Despite all this, the Maths is taught at a pace which (to me) is quite extraordinary - strong reasoning/ lateral thinking skills are definitely the order of the day. They definitely encourage independent thinking, rather than simply working to formulas. And they fly through the Mandarin as well.
Sounds like your DS would more than cope with all this though. Westminster is an amazing school with its own unique atmosphere and the results are pretty much second to none. Go with your gut feeling at the end of the day -and good luck!