I certainly wouldn't read the Oxbridge figures that way. At best they tell you what percentage of an average cohort is Oxbridge material. If your son is bright and motivated, then the figures you really need are the proportion of pupils applying to Oxbridge v those who get in (and as you get closer to that age, looking closely at success rates in the particular subjects). I would expect both schools to have a better than 50% success rate for those who do apply. But I would expect almost half of the boys at Tonbridge to apply for Oxbridge, with less than a quarter applying at Whitgift. The Whitgift intake has increased significantly over the last few years (up to 200 boys per year), and therefore the proportion of Oxbridge places in future may vary. I would also note that often Oxbridge isn't always the preferred destination of those who want to practice medicine (where London universities are often preferred), and the courses tend to favour those who want to go into medical research rather than practice (huge generalisation - but given this is a London school, there are many applicants who don't even waste an university application on Oxbridge as they favour the London colleges instead).
There is often some movement between schools at 6th form level, so don't assume that this gives you a true reflection of progress between 13-18 at either school. And don't be afraid to review your decision at this point in time either, especially if your son has a clearer idea of what he wants to study.
If you visited the schools on a non-open day then I'm afraid that the tour guides at each school will have been selected, with care. That is not to say that your guides were the only presentable boys in the school that day - far from it! But the schools know how to do a good sales pitch.
If I were in your shoes, I would want as much information about my son's cohort as possible. How big are the year groups? Where is my son placed academically within that year group (whether someone is in the top third or the bottom third can impact their self-confidence - asking which maths set he will be placed in will give you some good info))? What are the boys in the boarding house like, and what are their standards in terms of academics (eg at DS1's school the boarders have a higher GCSE results targets than the day pupils).