I'm hoping GS will be a more rounded experience for DS than indie..which I'm afraid makes some people terrible snobs!
I think it depends on the school. The indies I attended were pretty mixed and laid back, the indie prep DC went to was pretty snooty (or rather, a section of the parents were), whilst the grammar DC now attend is less snooty than the prep although (or because?) the parents, on average, have more to be snooty about (but are mostly too nice to be so)!
Re the main point of your post, with a bright child and anxieties about payment, I'd go all out for grammar, but also investigate backups (I presume you've looked at scholarships/bursaries) just in case.