I know quite a few Oxbridge graduates and all of them are quite average. One's an accountant and trained with PWC, works as an inhouse accountant for a big pharma company, not a finance director or running the company, quite average, another works in publishing and earns not that much, tried to write a book but it amounted to nothing, another is a tutor for 11+, GCSEs and A-levels and also earns not a huge wage and the other tried to work in the media but that amounted to nothing and now is a SAHM. Another works in the civil service, again in an ordinary job, doesn't seem to take home a huge pay packet, not bad but nothing like what you could earn if you worked for say Facebook in coding.
I'm not sure what the fixation is with Oxbridge. Everyone I've ever met from there has amounted to just the same as pretty much everyone else I know...some successful some not so much.
My most successful friend ever went to Exeter, just for the record.
And friends who tried but failed to get in were equally as bright as the ones who were rejected. It was very hit and miss. Honestly not sure what the cult status is all about.
I'd go with Tiffin if you're worried about money. It's a very good school and you can take the kids skiing etc and have lots of fun outside of term time. There's not much in it in terms of results between KCS and Tiffin. Why put yourself under that pressure.
Perhaps OP went to oxbridge and feels history must repeat itself for DS to go too... I've seen quite a bit of that recently too.