Terragen doesn't generate erosion maps at all at present, and they can be quite handy, so GC2 or World Machine are great for that alone. They also both have more sophisticated erosion tools than TG presently has. The Alpine Fractal is not an erosion *simulation* like CG2 and WM both do, it is an approximation based on noise functions and some creative behind-the-scenes ideas of Matt's. So the results have the advantage of being procedural and thus "infinite" detail, you can zoom in to a centimeter level, or zoom out to see a whole planet and maintain detail the whole way. Heightfields are finite in size and resolution and so don't have that advantage, but they're more "directable", and the operations you can perform on them to customize the look of the terrain are different and in some cases more specific, e.g. different types of real, simulative erosion.
You can get somewhat of the best of both worlds by applying procedural noise functions over heightfield terrains. The Fractal Detail part of Heightfield nodes does exactly this, in a specific way for a subtle result that hopefully complements the base terrain rather than changing its character. But you can add all manner of other displacement to it, subtle or otherwise, and get different effects while still getting some benefits of the source terrain created in another app. The main thing to keep in mind is that the more procedural displacement detail you add, the less accurate and relevant your erosion maps will be, so I'd advise smaller-scale features only.
By the way, one great use for erosion map output from WM and GC2 is for masking objects like trees and bushes (think, for example, of how plants tend to stick to waterways, a stream flowing down a mountain will often have an area of greenery just around it), and for masking displacement for example Fake Stones (think of talus slopes, etc.).
- Oshyan
