I've recently visited both schools and was expecting to love Alleyns but I was left with the lingering impression that it was a bit rigid and cold compared to Rosemead. The pupils taking us around seemed very mature and perhaps a little joyless :/
Rosemead despite being mixed ability seems to have very effective strategies for stretching the most academic and supporting those with special talents. Its facilities aren't as good but it felt like the (much lower) fees were being spent in all the right places: MFL, music, art, sports, small classes with lots of TAs, technology enabled differentiated learning etc.
I think the biggest difference was that the children at Rosemead seemed articulate and confident but still more childlike and happy. It didn't help that one of the pupils at Alleyn's mentioned that a certain school leader is very strict / scary and shouts at the girls for having loose hair etc...
I'm just hoping the hear from parents at either school if my impressions are off the mark.