Although I don't have experience of boarding schools, I suspect private schools may be better as parents tend to be very sensitive to whether they are getting value for money - so if you're child is unhappy and/or under-performing parents want to ask why. Also private schools can be very quick to get rid of kids who cause trouble.

Our son goes to a boys only private school and I have to say its been a great success - but we did select the school by doing a review of all the people we knew who went to private school (not difficult in Edinburgh) and picking the one that seemed to produce the "nicest" most well-rounded people.