I don't know much about the IGCSE, but I do know that it's a knowledge test. The SAT on the other hand is an aptitude test.
If you're highly intelligent, you will do well on the SAT.
If you are an excellent student and you know a ton of academic stuff, you will do well on the IGCSE.
The IGCSE is probably easier to study for because it involves knowing facts, scientific procedures, and the like. The SAT can be studied for, but to a lesser extent.
Like I said, I don't know a whole lot about the IGCSE, but I can elaborate a bit on the SAT.
The SAT has three sections: Critical Reading, Math, and Writing. In Critical Reading there are about 18 or so questions that involve vocabulary. These are the most knowledge-oriented questions on the test. The rest of the Reading section involves reading passages and answering questions about them ("The tone of paragraph three is...."). The Math section involves mostly very basic math but some of the questions are difficult to comprehend and some are just plain tricky. The harder questions require more advanced math but topics past Algebra II are not covered. You may need to review some math concepts for the test, but most of the math is very basic so this should not take much time. The writing section is all about identifying sentence errors. There are about 10 writing rules that are tested. You probably all ready know all of them. But the Writing section doesn't test whether or not you know the rules but rather whether or not you can apply them. I know someone who got a perfect score on the writing section twice without ever memorizing the rules. He just read enough that he had a nearly perfect grasp of grammar and could "feel" where the problem in question number X was. The Writing section is statistically the most difficult section to score well on, but many say that it is also the easiest to improve on (Reading is commonly considered the most difficult to improve on).
If you want to go to college in the US, you pretty much need to take the SAT (or ACT). They might accept IGSCE scores, but they prefer SAT or ACT. Essentially all US students that apply to universities take the SAT or ACT. It is very rare to find one that did not.