I know a number of people who have DC in more than one selective Indie, and there are big differences. DS is at a school where A is the norm, and so there is a risk that a child dropping to an A in one subject might feel they had failed. Not surprisingly we have been very careful not to convey any expectations of our own, partly because I'm not sure if we care whether he gets all As, but also because peer pressure is enough. DD's school has nothing like those expectations. If she gets all A*s it will be because she has decided that is what she wants to do. Some will, but it will be a much smaller proportion.
It is not unusual for people with DC in different types of schools to say that each school is the right school for that child. There is also confirmation of homework loads on the lines described by OP, and that homework can dominate. In our case not a problem because DS seems to get on with it, probably slightly too quickly, and find time for sports, Skype etc, but he says others can get ground down. He also suspects that one or two have tutors to help them keep up.
I would not necessarily dismiss a hot house. However it needs to be the school that your daughter really wants to go to, because she will have to do the work. If she finds the academic atmosphere exciting then it is a great opportunity. She also needs to be the sort of girl who is not bothered if she is towards the bottom of the class - few will excel in everything.
I would not worry too much about whether she is bright enough. These schools dont often make mistakes, and when they do it is often because candidates are over-prepared and so appear more academic than they actually are. Summer term/holiday children are almost a year younger that the September kids and will continue catching up through Yrs 7 & 8.
A private theory - sorry those who actually know about education - but being good at maths seems to become more important in secondary. Bright girls at primary seem to be the ones who read and write well at an early stage. The mathematicians get their revenge at secondary when a logical approach helps in physics and chemistry, Latin and German. Even history, RE and English comprehension become more analytical.
Perfectly possible then for a child to realise half way through year 7 that they really are quite clever after all, even if they had spent primary mid way down the year group.
Assuming that your other choice has a good cohort who will do well, the question is really where your daughter sees herself, and where you think she will thrive most.