There's a reasonably mixed demographic in Sutton, although less than most central areas of London. All the Sutton grammars tend to have high numbers of East Asian ethnicities in particular, with relatively low numbers of White British families compared to the numbers in the surrounding areas.
Sutton is very much suburban. Personally I think that is a good thing, in striking distance of central London but not as busy/crowded. Most people have a car and drive far more. We walk to school and commute by train to work but use the car for clubs, weekend days out, going to the supermarket etc. Sutton Town centre has a bit of a bad rep but is improving. I personally like it, as it has a decent number of useful shops, not just lots of brunch places and overpriced gift shops like some areas I have lived in. There are some good parks/open spaces.
PP suggested house prices, which I agree with. Some areas e.g. near St Helier hospital have smaller, cheaper terraced housing. Other areas e.g. Carshalton Beeches have lots of bigger, spread out houses. Your money will stretch further in some places than others. Hackbridge might be a bit cheaper but on a good line to central, although possibly less desirable than other areas nearby.
Obviously they are boys schools. Sutton Grammar has a small playground and sports fields towards Cheam. They focus on football more. It is super well connected. Wallington has more space, and I think plays more rugby.
They are very opaque about score, but should tell you Waiting List positions. There is usually some movement, as some may go go private, plus some will be hoping for Tiffin waiting list offers. It will depend how highly your son scored in this test though obviously, and the 2 schools do things slightly differently. People say if you also qualified for Wilsons, you have a higher chance for SG & WCGS, but not sure if that applies to out of catchment.