SSAT / ISEE
The best way to determine whether to take the SSAT or the ISEE is to take full mock tests for both and go with whichever mock test you scored higher on. The SSAT and ISEE are somewhat analogous to the SAT before its 2015-2016 redesign and the ACT, respectively. The most important differences between the SSAT and the ISEE are:
SSAT has guessing penalty (-1/4 point for incorrect answer choice and -0 for blank question) while the ISEE doesn't
SSAT has more advanced verbal sections, while the ISEE has more advanced math sections
SSAT scoring has two verbal sections and one math section (in the test the math section is broken up into two parts, but for scoring it's treated as one section), while ISEE scoring has two verbal sections and two math sections
SSAT essay is "creative" and ISEE essay is "expository"
SSAT has at least twice more testing dates each year compared to the ISEE, which makes retaking the test easier to accomplish if necessary
Some schools prefer the SSAT while other schools prefer the ISEE or have no preference between the two. Even if a school prefers one test over the other, the other test may still be worth taking. For example, if a school prefers the SSAT but their preference is not too strong then the ISEE may be a better test to take if you score better on math sections than you do on verbal sections. The converse also applies: if a school prefers the ISEE but you score better on verbal sections, then the SSAT may be a better test for you to take.
My tutoring resources folder containing SSAT / ISEE book PDFs (email me)
I may recommend purchasing books with practice problems and exams due to the scarcity of high quality free SSAT / ISEE practice materials. For example: Official SSAT Practice Materials. None of the SSAT / ISEE books are perfect, including the official books, but several are decent.