That really depends on how much experience you have with 3d design. GMAX is a great program but it can be diffuclt to learn due to the complexty of it. Sorry I do not know that much about FSDS. As far as which is best it depends on who you talk to.
FSDS is payware while GMAX is free Which is why I chose Gmax I have nothing to loose if I can not get it to work But FSDS I do. I just did not want to spend any money on a program that I will never use.