Both gmatprep and powerprep are free. They are available for download from mba.com. Alternatively, you can request for a cd version when you register for the test.
I downloaded both, it didn't make sense to me to request for the cd and make my desk more cluttered than it is now.
Anywya, the powerprep mimics the desktop experience when you do a ETS test. The gmatprep is new and attempts to give you some desktop experience when you do the pearson test.
These tests should be used as part of your prep. Do one full test (don't skip the AWA!) and see which areas you are weak in. This will help you to narrow down your preparation. After a while, do anohter set and see whether you have improved or still need brushing up in some other areas.