jumpalicious. We had the same agonising decision last year: Tiffin versus a top all boys selective independent (we'd got the independent choice down to one school at that stage)
I hope this helps you, and any family looking at the same choice. I could have done with this last year..
We chose Tiffin in the end and it was completely the right choice for our family. We have experience of both independent school and grammar now within the family, so I can talk about the differences in general between private and grammar (in my experience), but also Tiffin specifically.
Really happy with Tiffin so far.
We've just had a parents evening. The teachers seem good and know my DS well (I did wonder whether they would in a class of 31 boys). DS in a class of super clever boys, but who also know how to have fun (lots of Xbox gaming during lockdown!). There are a few boys who are out of this world clever who stick out a bit, but the other boys accept them (my DS actually likes not being the cleverest kid in the class!). Lockdown provision has been excellent. It's quite a strict school with lots of rules and regulations, but I think the boys need that, and hasn't been a problem. Music, if you like it, is truly outstanding, and they encourage each boy to take up an instrument. Everyone has a chrome book for school and homework which works well (compared to the indie which doesn't specify which laptop pupils should have; consequently there is a lot of peer pressure, which I don't like). You would have to check the policy on this for the independent school you have to choose from. Mobile phones are not allowed on at Tiffin during the day.
The sport provision is great for a state school. They don't have the funds to pay for lots of ex-professional sports coaches, flashy sports facilities etc, but it has been more than enough for my DS. He is looking forward to getting back to post school rugby / cricket coaching and Friday afternoon games at their sports grounds in Thames Ditton (they bus them up there, but they need to make their own way home). There is also a brand new sports hall on the school site with updated changing facilities.
The PTA is really active - lots of motivated parents at the school, which is always a good sign.
In terms of choosing, it does depend on the family circumstances, the schools you are choosing between and, of course, the child . I personally could not (and do not) see c£150k worth (fees rise every year) of value in paying for the independent school for my DS versus Tiffin, but that was a completely personal decision and everyone is different.
You get a better customer service (teachers and staff on call any time, more or less) , smaller class sizes, shiner / better facilities with an independent school. In my experience the children are more spoon fed at an independent (my DS is motivated and wants to do well so doesn't need that). It is a nicer experience, but you pay for that. I try to avoid contacting Tiffin unless I really need to as I know they're so busy, but that's probably me. Any contact I've had, including form tutor and head of year, has been good. They are quite matter of fact and to the point, but not a problem. You definitely get more of a touchy-feely vibe at an independent school.
I love the fact Tiffin is mixed socially and ethnically. It has worked really well. My DS has a range of friends from across all backgrounds. I was worried at first as all of his peers went to comps or independents (only he and one other got a Tiffin place) so he didn't know anyone when he started, but because it is selective, lots of boys start without knowing anyone. The head said that the boys last year came from 100 different schools.
They do Latin as standard (which I was really keen on - you can also do Mandarin as an extra) and all boys do dance for two years, which, amazingly, my DS really enjoys. Drama is great too.
At a top independent I would expect you would get the contacts which may help in the future , but I think that this will become less of a thing going forward with the move by top companies to widen their access in terms of recruitment (this is happening already)
I would think the choice would be down to finances and attitude; some families have money from inheritance, bonuses etc they can spare, and others would only ever consider a private education for their DC and, if they didn't have the income, would make sacrifices for it. It depends on what you see as value for money - it's a very personal decision
If we hadn't got Tiffin, then we would have gone with the independent and tried to make it work financially. However, luckily we didn't need to and don't have that stress.
Happy to answer any specific questions. Good luck.