I htink we must stay realistic.

Big game based on terrain + paint trees, paint waving grass, solid editor that rocks : pick up another afffordable 3D engine indeed (UDK, Crysis, Unity, Esenthel , C4 engine, LE 2 , Torque 3D ? etc ...)

If you go for 3DGS , even with pro, it will lacks features in terrain , instanciation and vegetation tools ... stuff you can find on other 3D solution for huge terrain based game.