Every year students have this question, and most years the answer is the same: You can get by just fine with last year's version. That being said, the past 3 years' new editions have each come with about 10-15% new questions. GMAC is constantly refining their question pool, so newer questions are slightly better, but not enough to make a substantial difference in your score. If you have a version that is 1 year old, you're fine. You can study with the 2017 or even 2016 version and be alright.
The last exception to this rule was the 13th edition in 2012. This was the year GMAC replaced one of the essay questions with the IR section. This year, you did need to use the newest version.
The last exception to this rule was the 13th edition in 2012. This was the year GMAC replaced one of the essay questions with the IR section. This year, you did need to use the newest version.