I can only speak about TerraLand2. You do not get an indication of the end product until it imports the elevation data and the overlay image. The only thing that provides any hint is when you preview the area to be captured. Some satellite images are much sharper than others, and that is a fairly sound indicator of the end result. Fortunately, it only takes a few minutes to create a terrain with the image overlay, so you can try various resolutions to get the best result possible.
Since it is all done within Unity, the terrain is created immediately after the elevation data and image is downloaded, so you can see what you have.
Also with Terraland2, you can grab heightmaps from several sources, so you can try different sources to see which one will provide the best result. For instance, sometimes Google has the best satellite image, while other times Bing or one of the others is better. This is probably the best of the options that I have seen for creating heightmaps. They aren't as accurate as using lidar data, but it's what we currently have to work with.
The downside of Terraland2 is it creates a terrain height of 8848 for all heightmaps which is totally unrealistic. I have to use Shimon tool "LowerTheRoof" (See sticky in the CF forum) to set a more realistic terrain height without messing up the elevations. It only takes a minute or so, but it is an additional step.
If you have any courses you want to try, let me know and I will see what I can do,