Interviews have more weighting. As far as I know, at Cambridge the order is interviews, reference/grades, personal statement last.
On the actual question itself, I'm not commenting. Yes, A's worked hard and done damn well given his/her circumstance, but equally, B's made the most of what's been offered to them. (What would B have to do in order to impress, given your assertion that work experience is sorted for them? Which, for the record, it rarely is: the schools give details of placements to those who are interested, you still have to write off with CV and cover letter. Granted, you're in a position where you get told of them rather than seeking them out, but the idea that they're delivered is ludicrous.)There is absolutely no way in this instance of gauging who deserves the place more on the basis of grades vs. circumstance. Furthermore, the issue about IQ is almost redundant, given that we know people's IQ in different areas (verbal vs. spatial) etc. can differ wildy.
Finally, the interesting thing that's been done is we've taken the absolute two extremes. In the other thread, the automatic assumption was private/grammar school = brilliant teaching, probably middle-class background, optimal environment. There are kids at private schools who have a hell of a lot of family problems, there are children at comprehensives who are very well-off and have supportive parents. You also have children from deprived areas who are fortunate to attend top private schools on scholarships and still face the Bristol discrimination.
I don't like where this survey is going, because it looks like we're going to build up this ideal, stereotype of an image, then attack that for all it's worth, rather than the situation at hand.