You get photorealistic renders with TG, yes, all the times no, because the free version is limited in render size and AA settings, and with Terragen, the bigger the render, the better the quality. The TG licence is 99$, so it's pretty cheap anyway. But it gets a tad boring after a while. I was fed up with creating  terrains, water, atmospheres ( great ones, though), and nothing else. Too limited as it is right now. And terrain creation in TG relies a lot on 3rd party apps, free and for a fee. ( GeoControl, WorldMachine, Terramaker...)