I go to St Doms (A2) and we have brilliant teaching and great support for Oxbridge. All teachers only teach a-level (or btec lol), I don't know if CVSF do as well but I think the presence of all the lower year makes their teachers less specialised for a-level
Easy to get in if you're catholic.
Social life is what you make of it, segregation isn't that much of an issue (a lot of people come from the partner schools and so are already friends, and they're often of the same ethnicity)
Our maths department is very good with very good results (don't know how they compare with CV) (50% get A at AS, even higher I think with fm)
Econ department is also very good (also 50% get A at AS)
Work ethic is quite high and grades are reasonably high (though i think CV grades will probably be better on average)
A lot of the partner school students (those who got in easily) get kicked out first year, lol.
I would think you have a lot more independence at Doms, it's a nice experience and IMO better than a high school. You will make friends easily don't worry.
We haven't really got a common room, but a nice outdoor area and canteen (in a way it motivates you to go to the library or other study areas lol).
Also, we have biometric registration which means you use your thumb on a scanner to register for a lesson which is pretty nifty, though I doubt that will be the deciding factor between the two schools lol