Making terrain, options ??

Hi all, I am wanting to represent a HUGE area 300 km x 300 km. The area doesn't need to be a exact presentation of any real world area BUT I would like it to have semi realistic textures, they could be repeated if necessary . As my game is a flight simulator type game most of the terrain would only be viewed from above, so the height map doesn't need to be mega detailed, I just want the ground to look realistic (ish) from above.
So what are my options for achieving this, I've been looking at world composer and terrain composer in the asset store (not sure what the difference is between the 2 ) and other 3rd party apps but I'm just really not sure what would work best for my needs.
Ideally the program would be reasonably easy to use.