I use Softimage alongside Terragen. From a usability and tweaking perspective, anything that's moving that isn't water or the camera is often simpler to add in your 3d software. I sure as hell wouldn't attempt a full on animated forest in Terragen, but know ways I'd approach the same thing in Softimage, perhaps using Terragen to give me the landscape onto which I place my instantiated foliage, trees etc.
That's the way I'd go.