We're really tossing up. I think it will depend on the schools we look at when that time comes. Myself and my partner were both educated in single sex private/catholic schools and both found it great.
I've always been of the opinion that a great education should be free in this country and you shouldn't have to pay lots of money for one. I think it really does depend on the area where the school is, and the reputation. I'd also like my children to associate with all walks of life.
Having said that, i'm pretty sure that we'll choose private. If we are only able to have one child the we'll most likely send them to a private grammar school. If we have two or three kids (unlikely for us being able to) then we'll go for a catholic primary/high school. Sometimes it just comes down to cost, our first baby has cost almost 20k to make, so if we have more, we might not be able to afford a really expensive school.