I know people who have (or have had) children at both, and have heard both good and bad about both. Redland Green obviously has a more uniformly middle class intake so you would expect it to get better results; some people have said this, and the fact that they have been so oversubscribed from the start, means they sometimes seem to think they don't have to try very hard... One friend withdrew his daughter because they were offering little to no support for her fairly severe dyslexia (she's bright so was just about keeping up with average/expected levels, but was capable of doing much better with proper dyslexia support).
Cotham gets good results considering its very mixed intake, and has a very good reputation for music, dance and drama, if your DCs are into that sort of thing. But I have heard some occasionally worrying things about bullying/gangs/violence - though I guess you are going to get some of that at any large inner city school.
Possibly more worrying for me is a friend whose yr 9 son is extremely bright but is currently getting into trouble quite often, largely (it seems) because he is bored in lessons - they only seem to set for maths so far, and the level of some other things, like science, doesn't seem to be challenging him. He also seems to be influenced by the 'not cool to work hard' ethic while trying to impress the cool kids. I've heard similar from another friend whose son has now left Cotham. But again, that can happen in any school - it can be a question of individual attitudes rather than the overall culture of a place.
Have you been to look round and get a feel for the two schools?