Thanks! You can download the unpopulated version of the world now at my blog or the first post :)
I've never used those before and haven't actually heard of bryce 3d before somehow. I picked world machine as my main tool because it's fairly simple to use and allows me to make terrain paint masks. I've even learned how to stitch new parts to my maps (The mountain range in Bridgewood looks nothing like the original heightmap in WM).
I really want to know what The sims team use! I've asked before on twitter but didn't get a reply. :(