As pointed out above, every app has its pros and cons. Terragen has a highly realistic atmosphere model, especially for volume clouds, very good handling of water transparency and crisp antialising. It also has a very realistic model of material distribution on terrains.

Using TerraPak and especially understanding its tutorials are a good step towards more realistic results. I have also discovered that Geocontrol 2 could be essential : it creates realistic terrain shapes but also has a selection system that is a great improvement on the Vue one.
In Vue, you can select terrain areas with altitude, slope and orientation. Geocontrol adds roughness. You can also select areas affected by erosion filters. This really helps to place materials realistically in your scene.
Check out the image below. Notice that the rock texture is only applied to the "rough" areas, thus making it more realistic than if it had been selected with slope  only. This is a Geocontrol terrain and I used a selection based on roughness to handle material distribution.

I am still not sure that you can achieve the same realism with Vue as the images it produces have a more "illustration" that "photographic" touch, at least the ones I am able to make.
But I once discussed over landscape 3D app choice with Luc Bianco, who is a master Terragen user and he told me that Vue was one to watch as it was evolving very fast.

Fantasy pictures, free 3d models, 3d tutorials and seamless textures on Virtual Lands.