They're two very different workflows, so unless you're equally comfortable with both, or more willing to just learn however the "best" app works, then it might come more down to user interface. I wouldn't say that if the two were not fairly well matched. World Machine is is a superb product and my personal choice, but like Terragen it's more procedurally oriented (and heavily uses nodes, even more so than TG really). There is some good ability to create specific shapes and edits (e.g. spline tools), but it's not "painting" a terrain. Geocontrol is very versatile and does have some nice erosion (though I think some combination of WM and GC erosion is really the ideal as GC misses some things WM has in this area too), and it's much more capable in the area of "painting" what you want; it's kind of less technical, more artistic, in a way. Personally I couldn't really get used to its layers interface last time I used it, but I'm sure it's improved since then. Bottom line I think you should try both if you can and if the UI one of "clicks" for you more than the other, then by that one. Feature-wise they're fairly well matched.
- Oshyan