Originally Posted by docbert

But given that availability is exactly the same for GPU v's miles+copay, how does saving the GPU make sense?

GPU expire. Miles (+cash) do not. Use the GPUs first, and if you run out, switch to the Miles+copay.

The exception would be if you're on a high fare, in which case burning the miles (with no copay) might make sense, and keep the GPU for when you're on a lower fare.