Both Northlands and St. Andrew's are good schools and would provide the IB curriculum for your students to be admitted to European universities. Other schools such as Belgrano Day School, St. Brendan's follow the University of Cambridge CIE programme which would also provide a suitable international curriculum but if you are thinking of a non UK university the IB programme would probably be more suitable.
Forget this nonsense about the teachers not being 'posh' etc. An advert in an international journal attracts many experienced applicants and schools will select the best candidate based on job requirements. When a bilingual school appoints contract teachers they do so to fill key areas where they are in a position to boost the overall level of education of the school. You are likely to find experienced contract teachers in positions such as Head of Science or Head of History etc.
St. Andrew's probably has the better IB results (though I have no statistics to support this claim). The primary school is separate from the the secondary school and they are in the act of moving campus so I would enquire how this is likely to disrupt school life.
You will find a lot of people on this board give uninformed advice on where to live and what school is best based on their financial position and not on the reality of the matter. I would recommend looking at a 'prestigious' private school (like the ones you mentioned) where your children will have a full sports and academic curriculum as well as opportunities for a range of extracurricular activities such as music and drama. It only seems 'snobbish' if you can't afford it.
I was being ironic. Sorry if that was not clear. My point was that these schools are indeed snobbish and the hiring of teachers from England is an attempt to give an illusion of sophistication, i.e. they want to project an image of being like English "public schools". The teachers they hire are seldom graduates of these sorts of schools to begin with but in Argentina image is everything. I agree that the level of education at a school like Saint Andrews is higher than in most other schools but I'd also stress that there will be a lot of social pressure. If an international program is necessary, look around. Saint Andrews and Northlands are not the only schools. There are likely others that are less elitist that could offer what you need but in an environment a bit more normal.