OP, to your question about opinions on single sex vs coed education for boys, this is covered thoroughly in Leonard Sax's book, Boys Adrift. It is a good read for parents of boys for reasons that go beyond just boarding school. The author doesn't pull punches and names names of some schools, both pro and con.
That said, I've found that a parent may have various opinions and preferences, but it is also important to listen to one's child. DS is now very happy at a coed school. Depending on your boys, I wouldn't be at all surprised to find that different schools would suit each one.