rktb, shame we can't do coffee this week! But keep an eye out for our NE London meetups -- a group of us had dinner at the new Yumyums a few weeks back, and it was a good girls' evening out. 
In the end, both Dallington and Cavendish are good schools, but perhaps suit some kinds of families more than others. I think you need to ask yourself what you want, as well as trying to second-guess your dd's development.
For example in making our decision, I know the thing that pisses me off in my children's state school is that fact that it's all a bit slack and half-cooked. They start up an after-school activity, but never really follow it through. The maths is okay, but the teacher needs me to point out that my ds's maths skills are significantly below his English levels. I point out that the reading books are demotivatingly easy and only changed every two weeks, and there's a half-hearted attempt at ramping it up a level, but after a few weeks it all fizzes out.
So if I'm paying a high four-figure sum for my dd2's education I want a school where I know that I don't have to expend significant quantities of brainspace making sure that my child is being appropriately taught and challenged. I'm not an insanely pushy mother, but both dh and I have trad academic backgrounds and it's important to me that my children are in a reasonably structured environment with competent experienced teachers who share my priorities, and who don't have their hands tied by the national curriculum.
So personally I'd choose Cavendish over Dallington because none of my children are (ahem) lacking in confidence, and I know the slightly more laid back ethos of Dallington would make me twitchy (which is my problem, clearly, but then I'm choosing a school that I have to live with as well).
But if your priority is making sure that your dd has warmth and one-to-one attention, if you can keep your inner pushy parent under control (which I would struggle with) and if you instinctively prefer Dallington's slightly urban, bohemian vibe over Cavendish's more trad private-prep schooly feel then that's a perfectly legitimate basis for a decision.
In the end your dd would probably thrive in either, so it's important to make sure that your heart is okay with any decision you make as well as your head.
Don't go away without letting us know your final decision, will you? When do you have to let the schools know?