if you need really huge terrain, you would need a 3dgs pro licence, or can choose some other engines supporting terrain lod and chunk streaming (mesh and textures) by default, like Esenthel and UDK. both are cool, and free until you release a commercial product.
in 3dgs, below pro, it results in a performance drop, but can be okay if the overall level is designed well, and you use lod carefully. or you can use multiple terrains attached together on edges.
imo in my editor single terrain based level creation is easy, and most settings can be adjusted in real time. it is designed rather for the RTS genre, but can be okay for other games, if you learn its handling (okay I should update and finish the documentation).
but anyway, what kind of game do you want to make?