Mac programs (when on a Mac) tend to have a faster rendering speed making it easier to pump out videos efficiently.
Windows programs - such as movie maker - tend to have a greater flexibility and are often easier to work with.
Sony Vegas is another great editing software that is almost an advanced combination of the above.
Ultimately the quality comes down to the content used and the user.
The workability and output of the iMovie makes it superior to everything except programs as high caliber as sony vegas pro, adobe, etc etc def. better then final cut pro