Really depends on your experience in coding. I have dabbled in both, and Ubot is certainly much easier to use straight out of the box. Zennoposter is a bit more difficult to learn, but I think it also has a bit more potential.
Also, resources about using Zennoposter can be difficult to find. The thing that sold me on Ubot was their forum and how quickly questions would receive good answers. I don't know about you, but it completely drives me insane when I hit a problem and can't find a good resource for solving it.