It's not really about which is the 'best' school (or better out of two) as either school will enable a child to achieve whatever they're capable of - if they want to.
There's a lot of understandable angst about choosing a school but so much of it is unknowable - how is your child going to change between the ages of 11 and 18? what will their cohort be like? will they find friends quickly? will they discover a love for a sport or subject they've not encountered before?
You just have to go on gut feel and throw in a few practical things.
Do you want co-ed? If so, there's no contest.
Do you want acres of playing fields outside your door? If so, there's no contest.
Which school is closer and easier to get to? By public transport and/or car? For example, for us, Latymer is pretty accessible by public transport but driving is terrible and parking pretty impossible.
In terms of results (though it's pretty marginal at this level) it would be interesting to split out boys and girls at Latymer; co-ed schools never publish this data (well, I've never seen it) so it's hard to tell whether for example girls or boys are doing better.
We know lots of children at both schools and without exception they seem to be making the most of their opportunities.
We chose the school that was closest to our house. It's made a lot of things very much easier.