So I bought the 2015 chemistry review this year from Princeton before the 2016 one was released. I was wondering if I should buy the 2016 edition if there is a big difference between those two, or if I should just use my 2015 book.
The 2015 book is fine
I understand the material is the same but will there be different test questions in the 2016 edition?
Probably, but the only questions that really matter are the ones that the College Board writes. 