Couple of points, mainly regarding Thornden versus Kings!
Of course, as far as I know you may have visited both schools 20 times and interviewed 40 parents/DCs at both
in which case you will have formed your own opinions!
BUT if you haven't: I think if you had a choice between these two, you need to look carefully at what suits your DCs.
Kings, to me, being ex-grammar school myself, felt just like a grammar. Very strict uniform, very rules based, highly structured (they have something like 12 bands per year in each relevant subject!) and -this is hearsay from 'insider DCs- there IS a cultural divide between the 'posh Winchester' DCs and the Stanmore council estate DCs. DCs come from miles around, some 'fleeing' the less desirable comps (I assume you're Toynbee catchment?).
Thornden is very middle class. It has a very strict catchment and 6 primaries serve it. The uniform is strict but very casual. They only stream in a couple of subjects, and, again in my opinion, the school feels very 'comprehensive'.
In terms of academic results, they are quite similar though Thornden seems to pip Kings but perhasp not by a decisive margin. What Thornden do do is get the boys through.
I looked very carefully at both but chose Thornden because I feel it serves all its DCs better. DS1 is clever and would be top or next set wherever he went. DS2 is a 'C' grader. He'd be very aware and discouraged of his middling to below middling position if he were in Group 8 or 9 of 12 whereas, with practically no setting, he'd be happier bimbling along!
Of course, the other issue is I suspect you'd have a lot of trouble getting all your DCs into the same over-subscribed primary in Winchester, a point I'm sure you already know! We had to split schools for luckily only half a term when we moved into this area for the same reason.
Hope that helps and wish you all the best.