The SSAT and ISEE are very similar, but there are small differences in structure and scoring.
Check out this infographic that compares the SSAT and the ISEE.
Schools may require one or both of these tests as a part of their admissions process, so the test(s) you will take may vary depending on the schools you are applying to. Preparing for one test will help you prepare for the other, however it is a good idea to take at least one full-length practice test for each test if you plan on taking both.