I have experimented with both GC and Grasshopper. Check this link for comparison (http://designplaygrounds.com/deviants/rhino-grasshopper-vs-generative-components/). To my limited experience, Grasshopper is much easier to learn than GC .

Revit or Project Vassari from autodesk is also another tool that has some limited capability for generative design. check this website (http://buildz.blogspot.com/).

All of these applications require some knowledge of scripting if you want to do very complex forms with parametric functionality. I still prefer to use MOI for conceptual design. It is fast , easy use and enjoyable. In my opinion you can probably be able to do some fairly complex forms specially with available scripts and two new added feature (flow and twist commands)

-sharif