"The most prominent difference between GMax and FSDS (aside from free versus pay) is that while FSDS is without doubt "easier and friendler" to model with, exporting to the sim is less friendly and easy."
On the other hand, *I* would differ with my esteemed modeller and argue that exporting to the sim from FSDS is friendlier and easier than in gmax, while conceding that gmax is the more powerful modelling program. But then again, I am biased, having used FSDS since it first came out, and while "conversant" in gmax, am still unable to reliably export from gmax to FS-X.