The main difference is that the IGCSE still includes some elements of the old GCSE: coursework, oral and practical assessment as well as exams. The IGCSE is offered at different levels, and some teachers think that it has more scope for more able pupils at the higher level.
But - opinions on GCSE and IGCSE exams are mixed: many more state schools are entering their pupils for the IGSCE exam because they prefer the mix of coursework and exams which is now no longer so evident in the new GCSE curriculum. There is also more scope for teachers to choose from a wider range of material.
Last year (2013), 78,000 pupils took the IGCSE English Language exam compared with 17,000 a year ago. Some school enter pupils for both exams.
Some teachers – and pupils- think that the IGSCE exam is now easier than the new GCSE exam, because of the content of the syllabus and the range of assessment.
As a parent, it is probably unlikely that you will be able to choose which exam your child is entered for. If the school offers both, then discuss your child's needs. If your child achieves higher marks with coursework and performs badly under exam conditions, then the IGCSE is an option.
But be aware that some universities have their own preferences. An increasing number are happy to accept International GCSE English, but some will not.
If your child is heading for university you should, even at this stage, research entry requirements on each university's website.