Agree that when you are looking at league tables and the differences are just a couple of % at GCSE or A Level, and you factor in that even then there maybe statistical reasons for the differences such as exactly what has been included, you cannot decide one is 'better' than the other based on that.
In the end, location is very important. Close is definitely better, a short commute is okay, but a lomg one is to be avoided unless there really is no other option. Regarding extra curricular and pastoral care, remember that when any school offers a huge amount, any individual child won't access it all - think about YOUR child and which things they are likely to get involved with. 50 rugby teams might be impressive, but if your DS isn't into rugby, it will be irrelevant. Ability to be involved with extra curricular after school is very much tied up with journey home too. Regarding pastoral care, it's hard to get a handle on this one. Schools all talk the talk and will have policies. A lot can come down to how good individual form tutors are - and there's no knowing in advance if you will get a great or a less good one.
I do have personal experience of both super selective state and indie school. In my view, the indie sees the parents as customers more. They will go out of their way to respond quickly to parents queries and to try to create timetables etc to suit individual requests. In most Indies, there is a wider range of abilities than the super selective grammars where competition for places is more fierce due to there being no fees. There are probably more super clever children in the Grammars and a smaller ability range. That said, indies are often good at squeezing the best out of their less able students, so there can be great value added, and a bright but not super bright child can often get into the Indie if they pay full fees, but simply won't get into the super selective. I think those bright, but not super bright children often get great 'uplift' from a good Indie.
In the end a lot depends on money. If the fees are a drop in the ocean for you, the indie will offer smaller classes and nicer facilities and yes here will be lots of trips and visiting speakers etc. In my view, these are 'nice to haves' and if you have loads of money and the fees are not a sacrifice, then go for it. Also if your child isn't likely to get into the Grammar, it may be worth pushing yourself on fees if the state Comp options are not good, because there's a bigger chance of getting into the Indie. If the state options are great,nthenndon no need.
If however, fees will be a struggle and mean no holidays for years etc and you have a Grammar option or a great Comp, I think it's a no brainer and the state option is the one. A Grammar will deliver the academic education and loads of extra curricular too, it might not be so shiny or polished, but it will do the job for FREE and those extra shiny bits are not worth £18k if the great academic education can be had for nothing and that £18k involves big sacrifice for a family. Loads of well educated and interested families send their kids to Grammars and good Comps,nowhere their children get a string of great GCSEs and A Levels and embrace a range of acitivities and go to great Unis and careers. Having interested parents is the key thing and going to a good school, regardless of if you pay or not comes next.
Personally, if I had a Grammar offer, unless I was very very well off, Inwould take it.