I say the same to everyone looking at new schools, whether independent, state, selective, etc. From my experience, look at the teachers - they're the most important aspect, not facilities, not buildings, not the head, etc.
We went to several open days. The striking difference was how the teachers reacted to parents and prospective pupils. In some, we could go into a room and basically be ignored by the teachers who'd just continue talking/drinking in a little huddle, apparently oblivious to those of us looking around and making us feel we were intruding when we asked questions etc. In others, the teachers basically accosted us the moment we walked in, immediately engaging with my son, showing him displays, talking to him, giving him worksheets with games on etc., setting him up on computer tasks, etc. In one school, the guy we later discovered was head of maths took him to a table and played a maths related board game with him. These schools made a massive impression on my son and it very easily gave us a short list.
At the end of it all, it was the school that was the oldest, darkest, fewer facilities, etc., that he chose, all because of the way the teachers engaged with us. They gave off really positive vibes. A few years later and it has proved to be the right choice. The kids have really good relationships with most of the teachers and that follows through into pastoral care, lesson time, extra-curricular activities etc. In his first year, he found himself sharing a canoe with the headmaster in the middle of a lake, the head insisting he be called "John" instead of Dr Smith for the day which really broke down barriers. It's meant that DS has had the confidence to talk to the teachers whenever he has experienced issues whether with the work or with classmates etc that has enabled things to be quickly resolved.
It's the teachers that make a school, not the shiny buildings or the expensive lab/theatre equipment. People are most important. My advice would be that if you don't get "friendly", "approachable" vibes from the teachers, walk away!