@butternut1 Congrats! I've posted above about our decision between the JDs, so have a read of that for our reasonings.
Re timings: both RCM and Guildhall have an official day of 8am-6pm. You can request an earlier / later start or ending time, though in reality most of the teaching - orchestras, choir, musicianship, chamber music - are fixed times so it's only really the instrumental lesson(s) that can change.
At Guildhall, most of the younger ones do c.9am - 3pm (a bit longer if they're doing a second study instrument too).
At both RCM and Guildhall there will be only a small number of Year 4 and 5s. I asked about this a lot before making our decision, and it was roughly the same at both - about eight-to-ten children across those two school years.
At RCM, the "orchestra" experience until secondary school is actually more chamber music - they work with 12 children of a similar age (ie primary school age).
At Guildhall, the orchestra for the younger ones is all the string players aged 8-13 (about 35 of them) so it's true that your DD would be working with teens. But it's a fuller orchestra experience. That's true of choir and chamber music too (it's with 8-13 year olds). But my DC loves that, and there are some lovely friendships amongst children of quite different ages.
(I presume the RCM choir is also with teens, but you'd need to check).
At both RCM and Guildhall, musicianship classes are taken only with children the same age.
We were a bit nervous at first that DC might get a bit "lost" amongst the older ones at Guildhall, but it's such a relaxed, friendly atmosphere that actually they love it, and has friends three of four years older as well as their own age.
But I do think, in the end, it just comes down to a gut feeling about which will best suit your child. Are you able to attend post-offer Open Days at both? I know the Guildhall one is the first week back after Easter - I think that's the best way to get a feel for the JDs. They're all fantastic, so it's just personal choice in the end. We were highly swayed by the instrumental teachers at Guildhall after the consultation lessons (we did two consultation lessons at both Guildhall and RCM and they were really the deciding factor for us - DC loved both Guildhall teachers, but wasn't so keen on either RCM teacher. Given that's their primary relationship, I would definitely be guided by that too).
Good luck with the decision-making!