Going from GCSE/IGCSE to A Level/IB is a big step and there is a lot of opportunity for error, but there are important things worth knowing that will reduce the possibility of a big mistake.
In general sixth forms are better than colleges.Sixth forms take 16/17 year olds straight from A Levels, but colleges take a much wider range of students and teach a much wider range of courses. This means that sixth forms are more focused on getting you through A Levels and on to University
There laws governing standards in sixth forms are more rigourous than the laws governing standards for colleges. Every sixth form is part of a school - catering to years 12 and 13 - and this means that teaching standards are higher.
Many students can go from GCSE to A Level within their own school, and this makes for an easier transition. Many of the new intake will be already known to each other
Most sixth forms and colleges offer the core English/Maths/Physics/Chemistry/Biology subjects. More exotic subjects can be studies externally, at another college or sixth form, or even taken privately, often only booking and turning up for exams. Any student studying a subject privately will need to be very focused and motivated and should bear this in mind.
You should look at the results of the sixth form/college for previous years. Some have very good track recoreds, and specialise in Oxbridge entry but tend to be private tutorial colleges charges tens of thousands of pounds a year.