Gmax and 3DS max are almost the same. The main differance is that 3DS max has the option to make particle emitters, Gmax doesnt. If you already have
3DS max paid for and whatnot, id stick with that. If you havent paid for it and are useing the trial version, take a look at the price of 3DS max and decide for yourself if its worth it. Gmax you can get for free but idk where.