What a lovely choice to have to make! My DD is at JAGS and abosolutely loves it. She's got a good mix of friends - both 'girly' and 'sporty' mix, who all seem to get on well. The teachers are really supportive and school lunches apparently delicious. We were worried that all girls may be a bit much for her but she's found her group of friends who like similar things. Homework levels are reasonable (and none during holidays!) and as your DD has gotten in, she'll have no problem with the academic level. They are stretched but just enough to make the work interesting. We have friends at Alleyns whose children are equally happy. I get the impression that they are slightly more laid back about academics... but then that's still a pretty high standard. Just go with the atmosphere your DD likes most... she will thrive at either.
Congratulations to your dd, what a great choice to have.
What are your dd's interests? I have one friend who chose JAGs over Alleyn's as she thought it would be better for her very sporty daughter. She felt that Alleyn's might put boys sport first over the girls. No idea if this is true but something to think about, and ask about if relevant for your dd.
On the other hand, Alleyn's has amazing music teachers and ensembles, and their drama department is excellent as well. I think these departments really benefit from being coed (I think JAGs may collaborate with Dulwich College for music and drama so perhaps this isn't a massive issue in the older years).
I think you will get a really good idea when you revisit; you can try to picture your dd at the school when you visit. Does she have a preference?