I haven't spent too much time with Terragen, and there are certainly those here (like agiel) who have clocked many more hours in Vue, but here are my 2 cents anyway...

With TG, you almost automatically get a great render no matter what. This is because many of the scene variables are quite limited. You can't add objects (people, houses or trees) nor is it possible to create multiple terrains and rescale/rotate and place them. Moving the camera around is a bit more difficult-- in fact it's a whole different way of doing things.

It seems to me, using TG is like going on a vacation with your camera. You create a terrain, then start moving around it, looking for a beautiful view and taking pictures. You tweak a few settings, including adjusting the terrain, materials and atmosphere, and you're generally guaranteed a great image. The downside is it's often hard to create the EXACT image you're after. TG does a GREAT job with textures on terrains.

On the other hand, with Vue, you can certainly create junk. Not every image is going to be photorealistic-- and you do need how to learn to tweak the settings to fix the aliasing versus render-time trade-offs. But, if you take the time to learn it, Vue can become a very powerful tool. As agiel pointed out, there are quite a few real masters out there who can create stunning images. The EXACT stunning images they were after.

HTH,
Chipp