Gosh. That's a tricky one. I realise this is the last thing you want to add to your list of things to think about but what do you want for them for big school? Boarding or London? This will greatly influence your decision
No point what so ever in doing garden house. (If a consultant suggested it, sack them) It has only recently extended its boys school up to 11 and was traditionally a school one left at 8+ - it would be madness to move back for a school that can't go to at least 13+
If you want the option of boarding or for your more academic boy (if that is how things continue to pan out) to go to westminster or St Paul's at year 9, you need one that goes to 13. However if you think your other boy is likely not to be so academic and you don't want to board, then you want him to be prepared for 11+ entrance. Also perhaps somewhere that would allow them to stay right through but have the option to change if they develop academically could make sense.
However , don't read too much into the difference in 7+ performance as it is a tough hurdle for little boys and a small difference in maturity can easily make the pass fail difference. Getting in to Latymer and Highgate puts that boy definitely in the frame for Sussex house or indeed westminster under/Colet If the other twin is close academically perhaps you should consider westminster or Colet as it can take them right through and are very good for music. Sussex house is the other v musical school in your list but it will only prepare for 13+
Wetherby gives you the most flexibility. It caters for 11 and 13+ and is increasingly solid academically (a third of the year leaves at 8+ for westminster and Colet but as its star has been rising the academic quality of those staying and joining at 8+ has too and it now has its senior school which whilst as yet an unproven quantity comes from such a strong operation that confidence must be high. Also it has in my view one of the best headmasters in town. This option is probably the one that covers the most possible outcomes. Downsides - they don't have their own outside space but they are still very sporty and hearty. Music not quite so central but they offer loads of clubs and activities so I am sure the so have the usual choirs and orchestras.
Harrodian whilst offering you the chance to go through I am a bit less convinced by as I doubt it will offer the academics at least one of your boys want. You would have to find out what success they have (indeed interest they have) at preparing them to leave at 11 or 13 plus for other more stretching schools. Lots of grounds and sport though. Plus a good spot to live for ease of access to Heathrow. (In fact from that point of view you plan to live in the west, so Colet , Latymer and harrodian would give you three different levels to aim at and there wouldn't be logistical problems in going to different schools if you were willing to split them as they are all pretty close
I don't think hill house is worth moving for. There certainly won't be problems getting in - its chief selling point is its size and ability to deal with london's ever mobile families but it is pretty sclerotic and whilst some people could say their recent problems are "just admin" they have had it coming for a long time. Their facilities also push the ever-cramped Central London space problems a bit too far
Last thing your probably need now is more names but you should also look at Thomas's Battersea Not a huge 8+ intake but they do take some and it is still well positioned for the heathrow corridor , preps for 11 and 13 and is coed.
But, given the buildings most London schools are in you need to talk to them individually about mobility ahead of time
Note that cut off dates to register for 8+ schools are about now
Sorry a bit rambling