All the schools you mention have mixed reputations, it depends which one you think your child will fit into best. Each school will have its own entry policy, I suggest you look at their websites once your child is in Y3 to see what the position is - no point in relying on hearsay on this.
Also don't be put off by a few rough looking kids outside the school. Unless you go private or to a grammar this is bound to be the case as the schools you mention are comprehensives. A lot will depend on whether you want a single sex environment.
My dc is at St Cs and it is a kind caring environment. Lots of middle class children from Southfields attend the school. Most of them do well, the main thing is how determined and hard they are prepared to work. You will also find that lots of parents pay for private tuition once you get closer to GCSEs. Ashcroft has an excellent and improving reputation - it is more science based and has a zero tolerance attitude to discipline which is not to everyone's liking.
Burntwood has a good reputation. It is a large school and I think it is 60/70 % muslim now due to preference for single sex education. I have friends with children at all these schools and at the end of the day there does not seem an awful lot to chose between them.
The only school I know nothing about is Southfields. This school was perceived as being mainly for non English speaking children and I have never been aware of any children with English as a first language going there. It does not appear to be marketing itself for English as a first language children but I may be wrong about this and schools change.