My son is dyslexic/ASD and we went for Briteschool. It is much smaller than Interhigh, but it has a lovely, structured old fashioned feel to it's set up that really suits DS.
Critical thinking along the lines of my old public school is much encouraged rather than the modern spoon feed the little darlings approach that has become so prevalent.
He's only primary, but I have to say that I am seriously impressed with the quality of the teaching he's getting. The lessons are well structured & recorded, which DS finds incredibly useful as he can listen again to any concepts he's found hard to get his head around the first time, at his own pace. The class sizes are small, and not age dependent, so DS can slot into the group that matches his ability rather than his chronological year.
First College does not guarantee that qualified teacher status staff will be teaching your child, something I felt was important to me. Though I understand that free schools, academies and private schools generally don't have to stick to this. Briteschool only has QTS staff.However it has a very nurturing ethos, which I could see being of great value for any child needing a self-esteem boost as part of a bullying recovery programme.
Interhigh does offer much more on the social front, but I want my son to do his socialising mostly face to face at local clubs etc due to his ASD, so that wasn't a key criteria for me. The lesson replay facility is far more useful to us. Interhigh is also the one with the best LA relationships.
I agree so much with the above comments on going with the one that "feels right" in terms of your school/parental relationship. It's such a relief to no longer feel like a PITA when I need to ask a question, & have the teacher just pick up the phone for a chat to me as soon as there's an issue iykwim.
My choice is the best for my own DS, BUT I can see exactly why it wouldn't suit other children with different temperaments. Just like with brick schools, each online school has it's own ethos, and you want the one that feels like the best fit for your child as an individual. Do pick up the phone and chat to the schools before deciding whatever you do.