i like bryce as a tool, but found the erosion and such really lacking in any kind of real form, it seems to be something that either adds or subtracts from whats already there, so you don't end up with realistic erosion... but i've only used it very little, and may be that i don't know what the hell i'm doing
as far as worldbuilder, i did download it upon su_liam's mention and tried it out, and well, its a little daunting, as far as trying to stick all the modules together to make something coherant... probably a few tutorials are in order...