As far as their output - terrain quality, types of maps, etc. - I think they're fairly comparable, though I personally (slightly) prefer World Machine's general terrain look. GC's always just seems a bit rougher to me. Ease of use is probably the biggest differentiator, or rather I should say *style* of working. WM uses a node-based approach similar to TG2, so if you're used to that method for building a terrain, you should find it very intuitive, fast, and effective. GC uses an interesting mix of painting and procedurals, so you can more easily define explicit terrain shapes, and if you're comfortable with more painting-like methods of terrain creation it may be preferable. Personally I like WM more, but again it's personal preference. Finally in terms of cost, WM has a slight edge unless you need Pro (which most people don't).
- Oshyan