I'm assuming you are writing your desktop application for the Mac because you are considering Cocoa, if you're starting from scratch that's an excellent reason to pick a platform independent library.

That choice is one you usually make only once in the lifetime of an application, you'd better make the right one. And I'd widen the field a bit to include other options not limited to Cocoa and Qt. If you have a success on your hands you'll really regret not being able to easily support other platforms.

Let's turn it around, what compelling reasons would you have for choosing Cocoa over a platform independent solution ?