These schools are all fantastic, but very different. DS had a clear set of preferences: Westminster, City and then SPS. Others preferred SPS, and indeed Kings. I would take the cue from your son. He cant really make a wrong decision.
He has loved every minute at Westminster, and I suspect would have been less happy at the alternatives where he might have felt overshadowed by more assertive boys. He probably fits the Westminster stereotype described above.
I would support HappyGardenings comment "Interestingly my DH works with/for some of the richest and most successful business men in the world, who are usually based in London of course, many have DS's at St Pauls to date (last 5 years) none so far at Westminster. " Westminster appears to have a more diverse and less City dominated parent body.
Westminster offers scope for boarding, useful for those with mobile careers, but also great for those boys who want faux boarding, eg breakfast, sometimes dinner and school and sport on Saturdays. City in particular appears to close its doors quite early with little scope to stay on at school to revise for exams together or go to the gym.
Girls in sixth form inject a real energy and contribute to the sense of the sixth form being half way to University.
Team sport is inevitably limited at Westminster, in part because of facilities, but also because few choose the school for its sport. This means that there is less competition for places in the team, and they do less traditional sports: fencing, climbing etc, well. Boys who had not considered themselves sporty can find a passion, and there is an enthusiastic round of inter-house sports competition, where participation rather than skill is the key. (Inter house netball...) Music, and access to the Abbey, are wonderful.
Worth noting. City is significantly cheaper.