Well, iGCSEs are a lower level than A Levels. See, iGCSEs are the same as the British GCSE which is level 2 (? Someone correct me on this if I'm wrong) and A level are the level above, which is level 3. It really depends on how old you are and what qualifications you've already got. If you have any GCSEs or equivalent then you should do A levels
They both take 2 years to do. For the sciences the practicals are pretty useful, but not necessary - but you will be asked how to proceed with an experiment and why it works.
Most people take 9 iGCSEs - including Maths, English (lang and lit) and science (BTEC, core, additional or triple science - most IGCSEs are triple science) and 4 options. Most people then go on to do 4 A levels unless they are really clever
Also, I don't know how good your english is, but your grammar, spelling and punctuation is very important. In some exams, such as history, english and geography, up to 10% of the marks go towards those elements.
Hope I helped!