About the other part of the question:
BillG Why would I want one over the other?
-If you want quickly create a random landscape "Inside Unity Game Engine" with limited control over the "character" of said landscape, go with "Gaia" plug-in.
-If you want to create a very specific "terrain", using geological simulations and precise artistic control, that you can use within any content creation system (Blender, Vue, Terragen, Unreal engine, Unity Engine (and within Gaia plugin itself), and most other 3D content creation tools), go with Quadspinner "Gaea".
-If you want "precise control" over your level within Unity game engine specifically, with some parts expedited by automated tools (for faster iteration), you CAN actually get both. All three DO work together, Unity, Gaia, and Gaea.
Hope this helps!