Hi Montlieu,
So it strikes me that amongst your 6 possible options, in order to narrow it down you will want to consider state v independent, and single-sex v co-ed. The former has the most impact on you, the latter on your son!
For most families the decision of state v independent is fairly straightforward - independent is financially out of reach. These will include many professionals such as teachers and salaried GPs. At the other end there are many very wealthy families who only look at independent schools. And then there are others in the middle who have to look at the sums more carefully. For many years the Sutton grammars have been seen as a good free alternative to the independent sector.
It is worth looking at the current educational landscape. There have been significant cuts to funding (mainly 6th form, but knocks on to whole school) in the state sector, and funding going forward will be flat. So in order to give any teacher a pay rise, another salary must reduce (for most schools 80% of the budget will be salaries). Where possible schools are getting the cheapest teachers. In London there is a severe teacher shortage in some key subjects, especially science, and the Sutton grammars are in Outer London, so some teachers will work a few miles in in order to get Inner London weighting, others will be tempted by the independent sector. You can look up the accounts of both Wally Boys and SGS online. Finances are tight. Both have increased numbers (as they are paid per pupil). It is likely that at some point they will have to go into a multi-academy trust (as the girls grammars have) in order to further save costs. Wally Boys is setting up a free school in Croydon (2018 I think, possibly 17) so that will be a distraction for the leadership. It may help the school finances. Worth keeping an eye on job ads too - certainly Wally boys was advertising for general humanities teachers, teaching Geography, History and RS, rather than a specialist. Of the 3 Sutton grammar schools, Wilson is the most secure financially.
At the same time pupil numbers have increased significantly ( Croydon has added an additional 76 forms of entry at primary level in the last 5 years - other boroughs will have done similar). However the number of independent places have not. In theory with greater competition it should now be more difficult to get into an independent school - certainly immediately after the banking crisis, it did appear that some schools were open to almost anyone who could pay full fees, but I think last year there were boys who got into Wilsons but not Trinity.
Whitgift and Trinity will both have new heads in the near future, which inevitably brings change.
If you have the option of a grammar and would otherwise have to make sacrifices, then grab the grammar place, at least until GCSE - you can make a different decision at 6th form when the quality of teaching may matter more.
Single-sex v Co-ed
The obvious point to make is that in a co-ed school somewhere between 40-60% of the school will be female. If your son prefers the company of boys, this tends to reduce his pool of friends. On the other hand if your boy enjoys the company of both boys and girls, then he won't find as many girls int he single-sex schools. So the singlesex schools do provide a wider pool of friends, from the very testosterone-fuelled, through to the gentle and quirky. Given the range of clubs at the schools, especially the independent schools, your son is likely to find similar minded boys. Boys at co-ed schools who lie at the extremes, may find that more difficult, though co-eds seem to be happy social places. But I think you have to look at your son, and what environment suits him. I have had my sons in different schools, one co-ed, one single-sex as that suits their personalities.
Other thoughts
They're all academic schools, so a bright boy will find peers at any of them. The independents will probably have a wider range of ability certainly on entrance, especially Emmanuel as it has the juniors coming up, though you probably won't spot this by GCSE. There is a fair bit of movement across all of these schools at 6th form, so a decision at this stage still allows for movement later. And as every such thread on MN states - do factor in transport, and give your son an easy commute - it will make a huge difference when GCSEs come round.
As you're holding onto 4 independent places, and will be waiting for grammars, it would be polite and kind to those on the waitlists to release the schools as soon as you have ruled them out.