Our friend, Dr. Drang, gives a high-level look at these two stellar calendar applications for the Mac. I use Fantastical and BusyCal on my Mac. The former is my primary calendar app (since I work for myself from home now my life is not nearly as rigidly full of meetings and appointments as it once used to be). And for the times when I need a different view of my week or month I bust out BusyCal.
So long as we’re on the subject: I use Agenda on my iPhone, and Apple’s native Calendar app on my iPad.