I wanted to return to this thread almost 10 years later and update!
We chose the new-build school and DD went through the entire school from Reception to Y6. It was excellent - by and large - being a new school. The head did indeed know every family, and DD knew the older year (eventually two older years) much better than she would have done in an established school.
Parents were really motivated to build the school they wanted to see, and the PTA flourished.
DD left the school a couple of years ago and has gone off to grammar, which was always our hope.
I joined as a Parent Governor, and am still there all these years later!
DD2 is now at the same school, has been there from their nursery and is now in Y5. The initial head has moved on but there has been the minimum amount of turbulence, and the school is a key community hub.
Were there any downsides to a new school? Not very many. The school has grown year-on-year - so from being literally 90 children when it opened, it's now over 500. So for those who loved the "village school" feel, I guess it feels a different beast now, but that was always planned. A bigger school means a bigger budget and better resources - so dedicated SENCO, pastoral care etc.
There was some noisy building work from time to time, but contractors were experienced with this type of site build and it was never unsafe.
The one major downside was that the school wasn't full to start with (e.g. 25 reception children per class) as a lot of people were unwilling to take a chance on a new school. This meant it soon filled up with children of parents who had either forgotten to apply for a school place, had applied for unrealistic school places, or whose children hadn't settled in their initial school - meaning there was a higher level of need in DD1's year group - both PPG and SEN than in subsequent year groups.
The major bonuses have been fantastic, brand new facilities and resources (like VR headsets and beautiful sports grounds), enthusiastic teaching staff and very little "we tried that once and it didn't work" or "we always do it like this" - so it's been lovely to really feel involved like you can make a difference.
It has been fabulous to be on the journey building a brand new community, and not for a single second do I feel we made the wrong choice. I hope this helps anyone else in a similar situation.