It depends what you mean by super-selective.
St Pauls gets better results and is comfortably within the top 10 nationally in most if not all league tables. As Mominatrix suggests, Latymer is comfortably within the top 100 in the same tables, often about half way down.
Last year's GCSE results and league table places are based on selection five years earlier when co-education had not worked its way through Latymer. Given the demand now, results should be gradually improving, though parents with children in more than one school in the area confirm that the St Pauls schools continue to earn their academic reputations and results.
Latymer however can be quite tricky to get into, as is Alleyns in South London, which I assume spurred the question Mominatrix posed earlier in the thread.
Mixed means half the number of places available to either girls or boys. A significant number of parents and pupils prefer mixed, including many Europeans who are not used to single sex education in their home countries. Latymer has a prep who reduce the number of places available. And Latymer has real strengths in drama, art, music and sport, particularly rowing, which add to its appeal. Boys places seem to be particularly sought after as there are few alternatives in West London for boys leaving, especially state primaries, at 11+.
Anecdotally the numbers applying for Latymer are higher than other private schools in the area, and not far short of Tiffin. St Pauls and Latymer will both probably welcome the small number of really super-bright, but further down the list it is perfectly possible to get St Pauls or Westminster and not get Alleyns or Latymer. Indeed some turn down schools with stronger academic records in favour of the broader range mixed schools. Equally it is possible to be offered a place at Latymer and not be offered Hampton, Putney High, or Godolphin.
In short if Latymer appeals and your child is reasonably bright, it is worth applying. To most people the results are more than good enough. An a prevailing West London attitude that Latymer is less academic and so can be used as a safe fall back, does not work. The same applies to Alleyns, whilst demand for Emanuel and the Harrodian appears to be growing. In the end it is not really about which is the better school, but which is the better school for a particular child.
In terms of "posh" it is hard to tell what that means any more in London. St Pauls are clearly more established, whilst Latymer is a former Grammar. Latymer is easier to access for boys in the state primary system, and will have fewer existing links with the central/West London girls preps as it went co-ed less than a decade ago. St Pauls pulls in children from a much wider catchment, whereas Latymer is more firmly West London. St Pauls probably has more money for bursaries, though Latymer does its best. St Pauls is the bigger brand name and is very sought after by those who belong in London's international financial circles. Latymer is more arty. St Paul's single sex approach is more likely to appeal to some groups, including Asians, whereas Europeans often prefer co-ed. All differences, though not not "posh" exactly.