Zbrush , if you consider the price : 600 Euros and not some illegal version , is a great deal : But not for terrain !
It's tool for mainly 3D characters , now hard surface geometry things.
So if you buy it just for terrain , it's a bad choice !
If you plan making lot of other stuff not only terrain, the go for it.

But yes Sculptris, is awesome like it has been said , i use it instead of Zbrush even if i have baught it in the past !
Sculptris is fast powerfull and allow i find lot more fine control for my taste
And you have great tools like Reduce polygons brush if you need to optimize your work !
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.... When ... when ... will A8 make like other indie 3D engines : Unity, C4D, Esenthel, LeadWerks, Torque 3D , Ogre 3D and free terrain editors etc ... and have a complete terrain/vegetation editor in the world editor : It's standard feature ... really !!!
Thats' really sad , really for people using A8 only.
I actually love A8 and it's scripts , the more easy to use engine (workflow could be better) !
But i use some other enegine due to lacking terrain when i plan a project using terrain ! That's like that , no choice !