1. Generally speaking, if you want to study MBA - GMAT would be the first choice; Any master degree except MBA, consider GRE (or LSAT, if it is in law);
2. GMAT doesn't need too much vocabulary - not like GRE, though the more voc. you have, it will definately help you in reading. Of course, reading is the fundemental requirement here.
3. Math is relatively easier in GMAT than it in GRE - but, none of them are very difficult. The part our Chinese people might feel some difficult are usually in the statistics/probability or some 文字应用题 - you shall be able to get those parts done by doing some exercises.