Hi IDreamOfMBA,
There's not much of a difference between the 5th and 6th editions of the
MGMAT books, but you might want to talk to Kyle or Dmitiri about the details. The GMAT2016 has approximately 25% new content in it (relative to the OG13) and it includes access to some online tools that you might find useful. The relatively low price of that book will probably make it a worthwhile addition to your study resources, but you don't have to purchase it if you don't want to.
Since it sounds like you're just beginning your studies, then it would be a good idea to take a FULL-LENGTH practice CAT Test; you can download 2 for free from
www.mba.com (and they come with some additional practice materials). If you want to do a little studying first, so that you can familiarize yourself with the basic content and question types, then that's okay - but you shouldn't wait too long to take that initial CAT. That score will give us a good sense of your natural strengths and weaknesses and will help provide a basis for comparison as you continue to study. A FULL CAT takes about 4 hours to complete, so make sure that you've set aside enough time to take it in one sitting. Once you have those scores, you should report back here and we can come up with a study plan.
I'd like to know a bit more about your timeline and goals:
1) What is your goal score?
2) When are you planning to take the GMAT?
3) When are you planning to apply to Business School?
GMAT assassins aren't born, they're made,
Rich