First sight, he only difference is that with the "Garbage mate" you have more flexibility to build a shape.
Another difference is that with the "Garbage mate" you have more precision. The "Garbage mate" works with pixels, while the "Crop" function works by percents.
And another difference that can be very important when compositing, is the place in the pipeline.
A "Garbage mate" would be processed before than a Crop. In FC, effects are processed before "Motion Effects" (center, crop, distort..).
If you can do it with cropping, avoid the "Garbage mate".
Sound advice. Many thanks. My quirk is that I'm trying to reproduce a jumpbox in a web page and the crop feature will not allow me to keep the dialog box AND have the jump box drop down...sounds like a case for garbage mattes!