My DD1 goes to The Grove and we love it. I don't know what the religious references other people were talking about from 2011 but we have never experienced much religious stuff at school. They don't even do harvest festival which I think is a shame. Their assemblies aren't religious as far as I know.
I think the big thing between the three schos is personal preference. The Grove is very popular at the moment and it has a small intake, only one class for each year, a small site and a strong sense of community. Yes, building work is taking place next academic year for 1 year which should improve the buildings but we won't get more space as its limited by location ( in the heart of Totnes).
I haven't heard great things about Dartington to be honest. Again, this could be due to only hearing bad news rather than the positive but what I have heard put me off sending my daughter there. Yes, it's a beautiful school and I am deeply envious of the Forest school stuff and the outdoor pizza oven etc but school is more than just buildings. Academically I've had friends have real problems, they say its chaotic and it's a BIG school.
It would never have worked for us to go to dartington as we live in the centre so The Grove is within walking distance but I didn't get a good feel for my child there.
I have heard great things about St John's in Bridgetown, their head is meant to be fab, they have forest school etc too. I think the most important thing is to visit each school and meet with the Head, get a sense of what each day involves for the children. My DD1 is so happy there, she says she never wants to leave and has loved every single teacher she's had - she gets upset each year at the thought of leaving her current class teacher!
Also, The Grove are keen on supporting yr 6 kids do the 11+ exam for the grammar schools and I think about 6-8 of the 30 kids in yr 6 are going to one of them this year.
I know people who have taken their kids out of Park as their child struggled in such a free, child-led environment, someone who took their child out of The Grove as they struggled in the more structured, goal driven environment and people who want to move their kids from Dartington as they think there isn't enough academic rigour and that its too chaotic.
Ok, I've rambled! Let me know if there's anything else you want to know! 