GMAT Strategy

My friend told me official guide 2016 has 25% more materials.

I already have 13th edition does it mean I'll face disadvantage if I study 13th edition.

Your friend is right that roughly 25% of questions in the 2016 Official Guide are new, but the new questions aren't testing new concepts. It's perfectly acceptable to use the older version, so long as you make sure you really understand the mechanics of the problems in that book. Once you've worked through the entire guide, and you want more practice material, my advice would be to pick up the question pack from mba.com: https://www.mba.com/us/store/store-catal ... ack-1.aspx