In the first six months of our stay in CH, we were sending our kids to International School, later we shifted both kids to local schools for two reasons:
1. COST - huge difference, as everyone knows
2. UNIVERSITY - upon checking admission requirements at ETH as well as EPFL, I realised that the students from the state run schools stand a much better chance than IB especially. f.ex. EPFL needs just 'maturite' for local school students (no minimum grade), but an IB student need 38/42 OR an entrance exam. http://bachelor.epfl.ch/admission-requirements

hankag's point re English good enough for US/UK/Canada is very valid though, and does cause me some worry.