It seems to me that Colleger might have had a bad experience at St Paul's. She is way off the mark, and frankly that's one of the dangers of anonymous posters on message boards. You have no idea whether their ideas, opinions and tastes have anything in common with yours.
People who post about schools usually fall into 3 camps: personal knowledge that makes them favour the school, personal knowledge that prejudices them against the school, and no personal knowledge but oodles of received wisdom.
It does worry me though that people can peddle 'opinion' which others might take as truth.
There are undoubtedly some pushy parents at St Paul's, but one-dimensional is the last word I would use to describe the vast majority of the boys I know. It's not in the least pretentious. The tutorial system is great. I'm sure there are some very pushy parents at Westminster and every other highly selective school too - you need only look on the 11+ forum to see the extraordinary lengths some parents go to to secure a place at this type of school.
I don't know Westminster all that well. Frankly it's too far from my house to even consider sending my sons there, when I have an excellent, forward-looking, fabulous school 5 minutes from my front door.
OP, don't listen to any of us. Go and see the schools for yourself. See what fits with you and your family and your son. You might not like either of them. Work out which is more convenient, in terms of your son's travelling and your travelling - sports matches, concerts, plays, parents' evenings. Where are your son's friends going to live? Can he walk to their houses? I'm sure there are odious products of every school. Talk to the boys there and see what you think. We're very happy with our choice.