i'm still learning Vue6 Infinite but i find that when i use some simple tricks using clouds, haze, fog and light combinations and by adding Geo Control and Chipps TerraPak  you  can make some realistic scenes.
the basics are fairly easy to pick up but getting into the advance part takes a bit of time i'm finding but i find the more you experiment the faster it sinks in.

you can find some free tutorials at the geeksatplay website.

i'm not familiar with Terregen so i can't comment on it.