Typically, steam cycle (Rankine cycle) powerplants are used when you have a heat source that is hot enough to run a steam turbine. For example if you are burning coal or natural gas.
ORC's are typically used when the temperature source is too low to run a conventional steam cycle. They can also be used as a "bottoming" cycle where the discharge from a conventional steam turbine then cross exchanges with the inlet of the ORC. This is called a combined cycle power plant.
More commonly, you may hear of a combined cycle power plant when referring to a natural gas turbine that is "stacked" on a steam turbine (the discharge from the natural gas turbine heats water to create steam and run a steam turbine).
So they are meant for different applications. The use of ORC's is growing fairly rapidly since the most common geothermal resources in the world are relatively low temperature (not hot enough to run a steam cycle).
Fairly interesting stuff, although I would say that it really isn't a DIY project.