I don't want to open up a league table debate on your thread but they are not to be trusted when comparing independent school and state school results. Often bright children in independents sit exams that count as a fail in the league tables or if they sit them early then that also counts as a fail. When I look at how good an independent school is I tend to look at their results A'level by A'level and not the overall score. I don't know how selective the indie is but if it is less selective than the grammar and still achieving outstanding results then it shows that the quality of teaching, in my opinion, may be superior. I also look at how flexible a school is e.g. if the average number of A'levels sat is three but there are a clutch of pupils getting 5 or 6 A'levels then that shows me that they are willing to go above and beyond to offer these very able children more teaching and resources. There are some well known very selective schools that only let a child sit 3 A'levels so its frankly not surprising if they constantly hit the top of the league tables as it is often based on % of A's.
If you have been offered a good scholarship and the school is closer then I would opt for that one and I would probably say the same if the grammar was very close and the indie was further away. When it comes to your decision it is probably coming down to very tiny pro's and cons either way, so if you are really struggling with what to do you could always justify sending her to the indie because not sending her to the grammar may mean you are opening up a place for a child who has no options.
I said some stuff previously in my pm about how they look after scholars, although they are not singled out, and I know this to be the case from my experience and I think it is also the case for some of the others on here whose kids are scholars.